THE  PRIi 
OFAEROGRAPEY 


THE   PRINCIPLES  OF 
AEROGRAPHY 


THE  PRINCIPLES  OF 
AEROGRAPHY 


By 
ALEXANDER  McADIE 

A.  Lawrence  Rotch  Professor  of  Meteorology,  Harvard  University,  and 
Director  of  the  Blue  Rill  Observatory 


RAND   McNALLY   &   COMPANY 

CHICAGO  NEW  YORK 


Copyright,  1917 
By  ALEXANDER  McAoiE 


THE   PREFACE 

Several  excellent  textbooks  on  meteorology  have  been  published 
in  this  country,  the  latest  having  been  issued  about  seven  years  ago. 
In  the  interval  since  then  much  new  material  in  connection  with 
the  exploration  of  the  upper  air  has  accumulated,  which  has  been 
published  only  in  scientific  journals;  and  it  is  thought  advisable 
that  an  effort  be  made  to  present  this  new  knowledge  in  a  convenient 
form,  even  if  considerably  condensed. 

Again,  the  student  of  to-day  is  interested  in  aerography  in  much 
the  same  way  that  the  student  of  geography  is  interested  in  his 
subject.  He  is  not  satisfied  with  merely  locating  places,  a  task 
essentially  mechanical ;  but  goes  on  to  trace  the  relationship  between 
physiography  and  the  development  of  communities  or  nations — • 
truly  an  educational  labor.  Thus  aerography  resembles  geography 
in  the  larger  sense,  while  meteorology,  according  to  the  general 
acceptation  of  the  term,  remains  the  science  of  recording  diverse 
atmospheric  conditions.  The  chief  purpose  of  aerography  is  explora- 
tion with  a  view  to  utilizing  the  knowledge  gained  to  insure  human 
safety  and  to  expedite  progress. 

The  present  book,  therefore,  aims  to  give  prominence  to  recent 
work  that  has  been  done  in  exploration  of  the  air;  such  work  as  that 
of  the  first  director  of  the  Blue  Hill  Observatory,  the  late  Professor 
A.  Lawrence  Rotch,  and  his  colleague  and  friend,  the  late  Teisserenc 
de  Bort.  Frequent  reference  is  made  to  the  work  of  Shaw,  Dines, 
Gold,  Cave,  Hergesell,  Assmann,  Koppen,  Sprung,  Suring,  Berson, 
and  a  host  of  other  modern  workers  who,  in  many  lands,  and  often 
under  difficulty,  have  contributed  to  this  slow  conquest  of  the  air. 

Another  important  reason  for  offering  this  volume  is  the  desire  to 
further  the  use  of  the  c.g.s.  system  of  units.  Throughout  the  book 
preference  is  given  to  absolute  units,  in  the  hope  that  the  student 
will  forget  as  soon  as  possible  the  old,  arbitrary,  and  irrational 
units.  Thk,  it  seems  to  the  author,  is  of  importance  for  at  least 
three  reasons:  first,  the  use  of  these  units  leads  to  clear-cut  concep- 
tions of  the  magnitude  of  the  changes,  regular  or  irregular,  in  pres- 
sure, temperature,  humidity,  and  air  flow;  second,  by  means  of  them 
much  time  is  saved  in  all  computations;  and  third,  they  lessen  the 
chance  for  error  in  observing  and  reducing. 


362^46 


iv  THE  PREFACE 

More  than  the  usual  attention  is  given  to  cloud  forms  and  the 
thermo-dynamics  of  their  formation  and  dissipation.  However, 
no  attempt  is  made  to  reproduce  in  extenso  weather  charts  and 
photographs  of  common  instruments ;  the  former  are  given  in  official 
reports  and  the  latter  belong  more  appropriately  to  the  catalogues 
issued  by  instrument  makers.  Stress  is  laid  rather  on  modern 
methods  of  attack  and  the  practical  application  of  whatever  knowl- 
edge is  already  available. 

It  may  be  pointed  out  that  the  book  contains  many  features  not 
to  be  found  in  other  textbooks.  Some  of  these  are : 

1.  Results  of  recent  aerological  investigations. 

2.  Introduction  of  new  units. 

3.  Cloud  classification  according  to  origin  rather  than  appearance. 

4.  Studies  of  air  flow  at  different  levels  and  the  gradient  wind. 

5.  Correlation  of  abnormal  seasons  with  hyperbars  and  infrabars. 

6.  Studies  of  ice  storms,  snowfall  equivalents,  and  water  supply. 

7.  Recent  floods  in  the  United  States. 

8.  Charts  for  aviators  (Rotch). 

9.  Variation  of  ocean  currents. 

10.  Recent  knowledge  of  solar  phenomena. 

There  is  also  full  discussion  of  thunderstorms  and  lightning  pro- 
tection; frosts,  and  the  best  methods  of  minimizing  the  injury  there- 
from; and  other  practical  problems.  The  author  has  tried  to  speak 
with  his  readers  rather  than  at  them,  fully  realizing  that  the  sum  of 
knowledge  is  indeed  small  as  compared  with  that  which  remains 
unknown ;  and  he  hopes  that  the  book  may  bring  a  certain  fellowship 
among  those  studying  the  problems  of  aerography  and  further  the 
application  of  knowledge  for  the  good  of  mankind. 

ALEXANDER   McAoiE 
Blue  Hill  Observatory 
Readville,  Mass. 
January,  1917 


TABLE   OF   CONTENTS 

PAGE 

List  of  Illustrations   .• ix 

List  of  Charts xi 

The  Preface xiii 

CHAPTER    I 
A  BRIEF  HISTORY  OF  METEOROLOGY 

SECTION 

1.  The  Dawn  of  Meteorology 1 

2.  The  Exploration  of  the  Air 3 

3.  Chemical  Composition  of  the  Air  Becomes  Known 6 

4.  Kites  and  Balloons 7 

5.  Determining  the  Height  of  the  Atmosphere 19 

6.  Distribution  of  Gases  in  the  Atmosphere 21 

7.  Molecular  Weights 24 

CHAPTER    II 
UNITS  AND  SYMBOLS 

8.  The  Centimeter-Gram-Second  System 25 

9.  The  Unit  of  Pressure 27 

10.  International  Symbols 31 

CHAPTER   III 
TEMPERATURE  SCALES 

11.  The  Nature  of  Heat 33 

12.  The  Measurement  of  Heat 34 

13.  Temperature  Scales 35 

CHAPTER    IV 
THERMODYNAMICS  OF  THE  ATMOSPHERE 

14.  The  Specific  Heat  of  Air -37 

15.  The  Equation  of  Elasticity 39 

16.  Dynamical  Heating  and  Cooling  of  the  Air      .  ...     41 

CHAPTER   V 
STRATOSPHERE  AND  TROPOSPHERE 

17.  The  Stratosphere  and  Troposphere        .  46 

v 


vi  TABLE  OF  CONTENTS 

CHAPTER   VI 

THE  CIRCULATION  OF  THE  ATMOSPHERE 
SECTION  PAGE 

18.  Effect  of  the  Earth's  Rotation  on  the  Atmosphere 54 

19.  Impressions  of  Gyroscopic  Motion 59 


CHAPTER   VII 
THE  MAJOR  CIRCULATIONS 

20.  Hyperbars  and  Infrabars 65 

21.  The  Effect  of  Ocean  Currents  on  Atmospheric  Circulation    ...      70 

CHAPTER   VIII 
THE  MINOR  CIRCULATIONS 

22.  Cyclones  and  Anticyclones 77 

CHAPTER    IX 
FORECASTING  STORMS 

23.  Types  of  Storms 87 

24.  Tornadoes 90 

25.  Waterspouts 93 

CHAPTER   X 
THE  WINDS 

26.  Wind  Systems 97 

27.  Trade  Winds    ....  98 

28.  Monsoons 101 

29.  Local  Winds 102 

30.  Cold  Waves  and  Boreal  Winds .      .      .104 

31.  Charts  for  Aeronauts  and  Aviators 105 

CHAPTER   XI 
THE  WATER  VAPOR  OF  THE  ATMOSPHERE 

32.  Earliest  Knowledge  of  Cloud  Formations 108 

33.  Measurements  of  Cloud  Altitudes 110 

34.  Classification  of  Clouds ...110 

35.  The  International  System 113 

36.  Distribution  of  the  Various  Types  of  Clouds 121 

37.  Wave  Motions  in  the  Air  Shown  by  Cloud  Undulations        .      .      .123 

38.  The  Value  of  Clouds  in  Forecasting  Weather  Changes     .      .      .      .125 

39.  Recording  Sunshine 131 


TABLE.  OF  CONTENTS  vii 

CHAPTER   XII 

CONDENSATION 
SECTION  PAGE 

40.  The  Formation  of  Clouds  and  the  Condensation  of  Aqueous  Vapor  137 

41.  Conditions  Present  in  Condensation 140 

42.  Formation  of  Fog  at  Sea 148 

43.  The  Dissipation  of  Aqueous  Vapor 152 

CHAPTER   XIII 
DUST  AND  MICROBES 

44.  Foreign  Matter  in  the  Atmosphere 154 

45.  Halos  and  Coronas 164 

CHAPTER   XIV 
ATMOSPHERIC  ELECTRICITY 

46.  Thunderstorm  Phenomena 167 

47.  Cause  of  the  Turbulenee  in  Thunderstorms .  173 

48.  Conditions  Favorable  to  Thunderstorms 176 

49.  Lightning 186 

50.  Other  Forms  of  Discharge      ....            193 

51.  Destructive  Effects  of  Lightning       ...             198 

52.  Protection  from  Lightning 202 

CHAPTER   XV 
PRECIPITATION 

53.  The  Process  That  Makes  the  Raindrop 205 

54.  The  Rain  Gauge 207 

55.  Variation  of  Rainfall  with  Altitude  in  Mountainous  Countries   .   211 

56.  Measuring  Rainfall  by  Rings  of  Annual  Growth 214 

57.  Rainfall  Distribution 217 

58.  Snow  Crystals 221 

59.  Measurements  of  Snowfall 223 

60.  The  Economic  Importance  of  Snow 225 

61.  The  Various  Types  of  Ice  Storms 230 

62.  Air  Temperature,  Rain  Temperature,  and  Temperature  of  Objects  231 

63.  Wind  Conditions  Which  Produce  Ice  Storms   .      .  ....    234 

64.  Formation  of  Dew  and  Frost 242 

65.  Dew  Deposits 244 

CHAPTER   XVI 
FLOODS  AND  NOTABLE  STORMS 

66.  The  Relation  between  Storm  Frequency  and  Floods        ....   246 

67.  The  Galveston  Storms 252 

68.  The  New  Orleans  Storm  of  September  29,  1915     .  .   257 


viii  TABLE  OF  CONTENTS 

CHAPTER  XVII 
FROSTS 

SECTION  PAGE 

69.  The  Relation  between  the  Surface  Flow  of  Air  and  Frosts   .      .   259 

70.  The  Various  Processes  in  Frost  Formation 260 

71.  Conversion  Table  for  Frost  Work 270 

CHAPTER   XVIII 
SOLAR  INFLUENCES 

72.  The  Source  of  Radiant  Energy 273 

73.  Variation  in  Sunshine 275 

74.  Measurement  of  Solar  Radiation 277 

Appendix 279 


LIST   OF   ILLUSTRATIONS 

PAGE 

Cirro-cumuli Frontispiece 

Pascal's  Experiment  at  the  Tower  of  St.  Jacques,  1648 4 

First  Ascension  in  America,  St.  Louis,  September  15,  1904 12 

Launching  Ballon-sonde 13 

Filling  Ballons-sondes  at  St.  Louis 14 

Dines's  Light-weight  Meteorograph 18 

Pilot  Balloon  as  Used  at  Blue  Hill 19 

Tornado  at  Lawrence,  Mass 93 

The  Great  Waterspout  Eight  Miles  off  Cottage  City,  Mass.,  August  19,  1896     94 

Waterspouts  at  Chatham  Islands 95 

Sunset  Effects  (at  half -minute  intervals) 109 

Morning  Fog  Rising.     Sea  Fog  Augmented  by  Radiation  Fog      .      .      .      .113 

Strato-cumulus,  Low  Fog 114 

Cloud  Changes.     Alto-cumuli 114 

Cloud  Changes.     Alto-stratus  (at  one-minute  interval) 116 

Instrument  for  Measuring  Cloud  Heights 125 

Plotting  Machine  for  Measuring  Cloud  Heights 126 

Edge  of  a  Cumulus  Cloud 126 

Cirrus 127 

Cirrus  Plumes 128 

Cirrus .    128 

Cirrus  Bands 129 

Cloud  Formations  in  Advance  of  Storm 130 

Cumulus  over  Mountains 131 

Campbell-Stokes  Sunshine  Recorder 132 

Pickering's  Polestar  Record,  Blue  Hill  Observatory 134 

Turbulence  in  Thunderstorm  Clouds  (at  four-minute  interval)    .      .      .      .175 

The  Growth  of  an  Electric  Spark  Discharge 186 

Streak  Lightning,  Stationary  Camera 187 

Streak  Lightning  (Sequent  Discharge)  Rotating  Camera 187 

Lightning  Discharge  through  Clouds 190 

Lightning  Flash 191 

Photograph  of  Lightning  Taken  in  Daylight,  July  10,  1912 192 

Spectrum  of  Lightning 194 

Spectrum  of  Lightning 195 

Spectrum  of  Lightning 196 

The  Oldest  Rain  Gauge 208 

Snow  Crystals 222 

Small  Snowflakes 223 

Structure  of  a  Snowflake 224 

Method  of  Studying  Snow  Cover  in  the  Mountains  and  Probable  Run-off     .   228 

ix 


x  LIST    OF  ILLUSTRATIONS 

PAGE 

Ice  Storm,  January  18,  1909 240 

Ice  Storm,  Blue  Hill,  January  18,  1909 241 

Ice  Storm,  February  18,  1910 242 

Ice  Storm,  January  15,  1912 243 

Miami  Street  Canal  Bridge,  Dayton,  Ohio,  after  the  Flood  of  March-April, 

1913 250 

Post-Office,  Dayton,  Ohio,  after  the  Flood  of  March-April,  1913   .      .      .  252 

Frost  Crystals 266 

Frost  Crystals 267 

Saturation  Deficit  Recorder 268 

Direct  Photograph  of  the  Sun 274 


LIST   OF   CHARTS   AND   DIAGRAMS 

PAGE 

Courses  Taken  by  Balloons  in  May,  1906 16 

Distribution  of  Gases  in  the  Atmosphere 23 

Convenient  Conversion  Scale  (F.  to  A.) 35 

Monthly  Values  of  Temperatures  ....  .  .  48 

Rotch's  Diagram  of  Height  of  Stratosphere  with  Latitude 51 

Annual  Variation  in  Height  of  Stratosphere 52 

Height  of  Cirrus  Clouds .52 

Height  of  Cirro-stratus  Clouds 52 

Height  of  Cirro-cumulus  Clouds 52 

Effect  of  Radially  Directed  Winds  upon  a  System  at  Rest 61 

Effect  of  Radially  Directed  Winds  upon  a  Rotating  System 61 

Relative  Motion  of  Radially  Directed  Winds  at  the  Surface  of  a  Rotating 

System 62 

Absolute  Motion  of  Radially  Directed  Winds  at  the  Surface  of  a  Rotating 

System 62 

Typical  Pressure  Distribution  and  Wind  during  a  Dry  Winter  Month  .  .  68 

Typical  Pressure  Distribution  and  Wind  during  a  Wet  Winter  Month  .  .  68 
Normal  Wind  Directions  and  Velocities  for  January  and  February  .  .  .71 

Normal  Wind  Directions  and  Velocities  for  July  and  August 72 

Typical  Distribution  of  the  Wind  in  Cyclones  and  Anticyclones  ....  79 

Typical  Distribution  of  the  Temperature  in  Cyclones  and  Anticyclones  .  80 

Typical  Distribution  of  the  Pressure  in  Cyclones  and  Anticyclones  ...  81 

Storm  Paths  in  the  United  States 91 

Paths  of  Highs  in  the  United  States 91 

Distribution  of  Cloud  Types r  120 

Distribution  of  Cloud  Types 122 

Distribution  of  Clouds  in  Cyclones  and  Anticyclones 124 

Moonlight  Record  during  Eclipse 133 

Moonlight  Record  with  Cloud  Intervals 133 

Record  of  Day  Cloudiness  for  Two  Months 135 

Adiabatic  Diagram 141 

Foehn  Wind 145 

Foehn  Adiabat 145 

Mountain  Winds  Dried  and  Warmed  in  Descending  ....  .  145 
Sea  Temperature  and  Path  of  Air  during  Fog  off  Newfoundland,  S.  S.  Scotia, 

August  4,  1913 -149 

Sea  Fog  Temperature,  Conditions  during  Fog,  August  4,  1913,  7  P.M., 

S.  S.  Scotia 150 

The  Original  Dust  Counter  (Aitken) .  .  158 

Temperature  Gradients  within  (C  L  D)  and  without  (C  K  D)  Cumulus 

Clouds 173 

xi 


xii  LIST   OF  CHARTS  AND  DIAGRAMS 

PAGE 

Thunderstorm  in  the  Making 180 

Ideal  Cross-Section  of  a  Typical  Thunderstorm 181 

Voltage  of  Air  during  a  Thunderstorm  at  the  Top  of  the  Washington  Monu- 
ment         200 

Voltage  of  Air  during  a  Snowstorm     .      .  201 

Heaviest  Recorded  Rainfall 215 

Types  of  Monthly  Distribution  of  Precipitation  in  the  United  States    .      .219 

Depth  of  Snow  at  Summit,  Cal 227 

Chart  of  Conditions  during  Ice  Storm  January  5-6,  1910 234 

Temperature  Record  during  Ice  Storm .   235 

Temperature  Record  during  Ice  Storm 239 

Drainage  Basin  of  the  Mississippi  River 247 

Precipitation  during  Flood 249 

Weather  Conditions,  March  28,  1912,  and  Precipitation  during  Following 

24  Hours 251 

Barometric  Pressure,  Houston,  Texas,  during  the  Storm  of  August  16-17, 

1915 253 

Paths  of  the  Galveston  Hurricanes  of  1900  and  1915   .  .    254 

Pressure  at  New  Orleans  during  Storm  of  September  29-30,  1915    .      .      .    255 

Change  in  Winds  Near  the  Storm  Center 256 

Rainfall  Distribution  in  West  Indian  Hurricane,  September  28,  29,  30,  1915  257 

Types  of  Inversion,  Winter  Type 261 

Types  of  Inversion,  Unusual  Type 263 

Frosts  and  Inversions,  Typical  Late  Spring  Frost 265 

Thermograph  for  Dry  and  Wet  Bulb  Readings 269 

Temperature  Scales 289 


•L 


THE   PRINCIPLES   OF 
AEROGRAPHY 

CHAPTER   I 

A  BRIEF  HISTORY  OF  METEOROLOGY 

i.  The  dawn  of  meteorology.  In  a  lecture  before  the  Royal 
Meteorological  Society,  March  11,  1908,  entitled  "The  Dawn 
of  Meteorology,"  Hellmann  pointed  out  that, 
while  as  a  branch  of  knowledge  meteorology  is  weather  lore 
as  old  as  mankind,  as  a  science  it  is  very  young. 
There  is  a  vast  store  of  weather  lore  in  the  Bible,  and  also  in 
Homer  and  Hesiod;  and  there  is  reason  to  believe  that  much 
of  our  weather  lore  dates  back  to  an  Indo-Germanic  source. 
It  is  different  from  that  of  Babylonia.  There,  atmospheric 
phenomena  were  associated  with  the  constellations,  and  a 
complete  system  of  consequences  and  combinations  was  estab- 
lished. This  gave  rise  to  astro-meteorology,  which  became 
part  of  the  Assyro-Babylonian  religion.  The  Babylonians 
had  a  wind  rose  of  eight  points  and  used  the  names  of  the  four 
cardinal  points  to  denote  the  intermediate  directions.  The 
Greeks  were  the  first  to  make  regular  observations,  and  as 
early  as  the  fifth  century  B.C.  parapegmata,  or  peg  almanacs, 
were  fixed  on  public  columns.  Chiefly  these  were  observations 
of  the  wind  direction.  Anaximander  of  Ionia  (fifth  century  B.C.) 
was  the  first  to  give  a  scientific  designation  to  the  wind ;  and  it 
still  remains  valid.  He  defined  wind  as  a  flowing  of  the  air, 
av£)jLov  zivcti  pvtiiv  aepos.  There  is,  however,  neither  a  Greek 
nor  a  Roman  word  for  wind  vane.  The  Tower  of  the  Winds 
at  Athens  probably  served  more  as  a  public  timepiece  than 
as  an  indicator  of  weather  probabilities. 

Soon   after  these   earliest   qualitative   ob  nervations   of   the 
weather,  came  the  first  that  were  quantitative.     They  led  to 


OF  AEROGRAPHY 

measurements  of  the  rainfall.  Such  measurements  were 
made  in  Palestine  in  the  first  century  A.D.  where,  as  Hell- 
mann  points  out,  ''the  great  influence  of  rainfall 
records*1*1  on  ^ne  cr°Ps  must  have  been  fully  appreciated 
at  an  early  date  and  the  results  of  which  obser- 
vations are  preserved  in  the  Mishnah,  a  collection  of  Jewish 
religious  books  apart  from  the  Bible.  It  seems  most  inter- 
esting that  the  amount  of  rainfall  then  considered  as  normal 
for  a  good  crop  corresponds  pretty  closely  with  that  deduced 
from  the  modern  observations  made  by  Chaplin  at  Jerusalem; 
whence  it  can  be  inferred  that  the  climate  of  Palestine  has  not 
changed." 

Hellmann  further  points  out  that  two  early  physicists,  Philo 

of  Byzantium  (third   century  B.C.)  and  Hero  of  Alexandria, 

describe  an  apparatus  which,  though   primitive, 

physicfsts         *s  essentially    a    thermoscope.     Hero's    book,   on 

pneumatics,    was    translated   between    1575    and 

1592  no  less  than  twice  into  Latin  and  three  times  into  Italian. 

It  was  studied  by  Galileo,  Porta,  and  Drebbel,  and  gave  to 

all  three  men  the  idea  of  constructing  a  thermoscope  and  to  the 

last  one  the  impulse  of  making  an  experiment  on  the  winds. 

The  Greeks  were  also  the  first  to  advance  theories  of  me- 
teorological phenomena,  and  their  philosophers  had  much  to 
say  on  such  matters.  A  great  many  speculations  were  set 
forth,  and  by  the  time  of  Socrates  meteorology  was  held  in 
low  esteem.  A  new  word  was  coined  — //ercta/jo/U^s — to 
designate  a  mean  babbling  about  sublime  things. 

A  century  later  came  Aristotle's  treatise  on  wind,  the  oldest 
one  now  in  existence.  Hellmann  says  that  Aristotle's  most 
Aristotle's  distinguished  successors,  such  as  Theophrastus 
treatise  on  and  Posidonius,  added  to  it  but  little.  All  text- 
books of  meteorology  issued  on  the  continent  of 
Europe  before  the  end  of  the  seventeenth  century  are  exclu- 
sively based  on  Aristotle. 

It  is  of  interest  to  recall  that  the  military  expedition  of 
Alexander  to  the  interior  of  Asia,  including  India,  brought  to 
the  Greeks  the  first  knowledge  of  the  monsoon  winds ;  and  that 
the  Romans  were  the  first  to  point  out  the  difference  between 
a  continental  and  a  marine  climate. 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  3 

2.  The  exploration  of  the  air.  Exploration  and  systematic 
measurement  of  the  various  levels  of  the  atmosphere  date  from 
the  beginning  of  the  twentieth  century.  Although  ancient 
writers  did  indeed  discuss  at  considerable  length  the  nature 
of  air,  including  under  the  general  term  meteorologia  all  that 
was  known  regarding  atmospheric  phenomena  and  much 
that  was  essentially  astronomical,  there  was  no  precise  knowl- 
edge of  the  atmosphere  as  a  whole  nor  any  conception  of  the 
various  motions  of  the  air  or  its  composition.  The  probable 
height  of  the  atmosphere  may  have  been  apprehended  by  some 
of  the  ancient  writers,  notably  Posidonius.  However,  very 
little  that  is  serviceable  has  come  down  to  us  from 
early  times.  It  is  said  that  in  the  eleventh  cen-  instigations 
tury  Arabian  geometers  estimated  the  probable 
height  of  the  atmosphere  to  be  92  kilometers,  the  computation 
being  made  by  determining  the  duration  of  twilight.  Five 
centuries  later  this  upper  limit  was  redetermined  by  European 
astronomers;  but  not  until  the  middle  of  the  seventeenth  cen- 
tury was  it  known  that  air  could  be  weighed  and  that  the 
atmosphere  exerted  a  certain  hydrostatic  pressure. 

That  such  a  pressure  existed  was  proved  by  Evangelista 
Torricelli,  working  at  Florence  in  1643.  Torricelli  tried  the 
well-known  experiment  of  inverting  a  tube  filled  T  .  UJ 

with  mercury  in  a  vessel  containing  the  same 
clement,  thus  demonstrating  that  a  column  of  mercury  in 
vacua  could  be  balanced  against  a  column  of  air.  He  was 
not  concerned  about  the  height  of  the  atmosphere,  though 
this  could  very  easily  have  been  determined,  approximately 
at  least,  by  multiplying  the  weight  per  unit  volume  of  mer- 
cury (13.596)  by  the  height  of  the  column  (76  centimeters 
at  sea  level)  and  dividing  by  the  weight  per  unit  volume 
of  air  (0.001293).  The  result  of  such  a  calculation  is  7,991 
meters,  which  is  the  height  of  the  so-called  homogeneous 
atmosphere.  The  instrument  now  called  the  barometer  was 
first  known  as  " Torricelli 's  tube."  The  words  "barometer" 
and  "baroscope"  were  introduced  by  Boyle  in  1685. x 

An   experiment   of   equal   significance   was   that   of   Blaise 

!H.  C.  Bolton,  Science,  April  3,  1903,  and  A.  Lawrence  Rotch,  ibid.,  May  1, 
1903. 


From  Tissandier's  L'Ocean  Aerien 

FIG.  1.     PASCAL'S  EXPERIMENT  AT  THE  TOWER  OF  ST.  JACQUES,  1648 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  5 

Pascal.  On  September  19,  1648,  with  the  help  of  his  brother- 
in-law,  Perier,  Pascal  demonstrated  that  the  pressure  de- 
creases with  elevation.  The  height  of  the  col- 

r  •  L   £  o  Pascal 

utnn  of  mercury  in  a  vacuum  tube  was  3  ponces 

(76  millimeters)  lower  at  the  summit  of  the  Puy-de-D6me  (a 
mountain  in  Auvergne,  with  an  elevation  of  1,463  meters) 
than  at  the  base.  Pascal,  who  had  been  detained  in  Paris, 
soon  afterwards  showed  that  a  difference  could  be  detected 
even  at  an  elevation  of  50  meters.  He  noted  a  difference  of 
two  French  lines  (4.5  millimeters)  between  the  readings  at 
the  ground  and  at  the  upper  balcony  of  the  tower  of  St. 
Jacques  (Fig.  1). 

In  1650  Otto  von  Guericke  invented  the  air  pump,  proving 
that  air  could  be  weighed  and  also  that  the  atmosphere  exerted 

pressure.     He  constructed  a  huge  water  barom- 

„    _-  .  Von  Guericke 

eter,    called     a    semper    vivum,     or     perpetuum 

mobile,  in  which  a  small  floating  figure  rose  and  fell  with 
changes  in   pressure   due   to   atmospheric   conditions.     It   is 
said  that  he  predicted  storms  by  means  of  this  instrument 
as  early  as  1660.     In  this  same  year  Boyle,  in 
England,  published  his  book  entitled  New  Experi- 
ments Physico-Mechanical    Touching  the    Spring   of  Air  and 
Its  Effects.     Other  volumes  followed;  and  in  one  printed  in 
Io65  there  is  given  a  detailed  description  of  changes  in  the 
mercurial  column  and  of  weather  conditions  other  than  mere 
temperature  changes. 

As  early  as  1597,  Galileo  had  devised  and  used  at  Padua 
a  crude  form  of  thermometer,  a  modification  of  _  ... 

which  was  used    in  medicine  by   Sanctorius   as 
early   as    1624.     The  modern  form  of  thermometer  did  not 
come  into  use  until  the  middle  of  the  century. 

Meteorology  owes  much  to  the  little  group  of  nine  Florentine 
investigators,  most  of  them  pupils  of  Galileo  and  known  as  the 
Accademia  del  Cimento.  They  made  essential  improvements 
in  both  barometer  and  thermometer.  Descriptions  of  some 
of  their  experiments  may  be  found  in  the  Saggi  di  Naturali 
Esperienze,  published  in  1666.  The  first  attempt  to  establish 
an  international  meteorological  system  of  observations  was 
made  by  Ferdinand  II,  Grand  Duke  of  Tuscany,  in  1654. 


6  THE  PRINCIPLES  OF  AEROGRAPIIY 

In  1650  Southwell,  then  president  of  the  Royal  Society, 
brought  to  England  the  first  thermometer,  a  small  one  of  the 
Florentine  type;  and  Boyle  made  a  duplicate  of  it.  Within 
a  few  years  thermometers,  barometers,  wind  vanes,  rain 
gauges,  and  even  dew  collectors  were  in  use.  Boyle  in  Eng- 
land, in  1662,  and  Mariotte  in  France,  in  1676,  discovered 
the  law  governing  the  compressibility  of  air,  or  of  any  gas, 
but  the  composition  of  the  atmosphere  and  the  true  nature 
of  air  were  not  then  known.1 

3.  Chemical  composition  of  the  atmosphere  becomes 
known.  Our  knowledge  of  the  chemical  composition  of  the 
atmosphere  dates  from  the  middle  of  the  eighteenth  cen- 
tury. Ramsay  in  his  interesting  volume,  The  Gases  of  the 
Atmosphere,  gives  in  detail  the  progressive  steps  by  which 
investigators  like  Boyle,  Mayow,  Hales,  Black,  Rutherford, 

Priestley,     Scheele,     Lavoisier,     and     Cavendish 
Lavoisier  -,      -.  -,  •   ,     -.    •       ,-. 

finllay  made   known  what   gases  existed  in  the 

air.  Lavoisier,  the  first  successfully  to  separate  oxygen 
from  air,  in  a  pure  condition,  was  also  the  first  to  show 
that  water  vapor  must  exist  in  the  atmosphere.  He  saw 
clearly  that  although  water  is  a  liquid  at  ordinary  temper- 
atures, it  can  exist  as  a  vapor  and  mix  with  the  free  gases. 
Cavendish,  that  strange  and  solitary  genius,  the  discoverer 
of  hydrogen,  to  whom  the  world  of  science  owes  much, 
published,  in  1784,  the  first  of  many  important  papers,  en- 
titled Experiments  on  Air.  He  stated,  as  the  mean  of  many 
analyses  he  had  made,  the  following: 

79.16  per  cent  of  phlogisticated  air,  or  nitrogen; 
20.84  per  cent  of  de-phlogisticated  air,  or  oxygen. 

These  values,  says  Ramsay,  do  not  differ  materially  from  the 
best  of  modern  analyses.2 

1  Briefly  stated,  the  law  governing  the  compressibility  of  a  gas  is  this:    The 
volume  is  inverse  to  the  pressure,  provided  the  temperature  remains  constant; 
in  other  words,  the  density  is  in  proportion  to  the  pressure.     A  later  and  more 
general  form  of  the  law  connects  volume,   pressure,   and  temperature.     It  is 
frequently  called  the  Boyle- Gay- Lussac  law,  and  sometimes,  "the  equation  of 
state."     It  is  of  fundamental  importance,  as  we  shall  see  later. 

2  These  give,  within  small  variations, 

79 . 04  per  cent  nit fogen  and  argon 

20.96  per  cent  oxygen 

after  absorption  of  carbon  dioxide,  ammonia,  and  water  vapor.  (See  p.  22  for 
more  detailed  statement  regarding  composition  of  air.)  Carbon  dioxide  is  also 


A  BRIEF  HISTORY  OF  AEROGRAPHY  7 

Rayleigh  and  Ramsay,  in  1894,  separated  from  atmos- 
pheric nitrogen  an  inert  gas  which  they  called  argon.  Subse- 
quently other  gases,  such  as  helium,  neon,  krypton, 
xenon,  and  niton  were  discovered.  These  gases 
exist  in  exceedingly  small  quantities,  averaging 
not  more  than  one  part  in  a  million  by  volume.1  They  are 
sometimes  called  the  noble  gases  because  of  their  apparent 
disinclination  to  unite  with  other  gases.  Niton  is  an  emana- 
tion from  radium  compounds.  Coronium  is  a  suspected  gas 
lighter  than  hydrogen  or  helium.  Its  existence  is  indicated 
by  certain  lines  in  the  spectrum  of  the  sun's  corona.  The 
name  geo-coronium  has  been  used  for  the  extremely  rarefied 
medium  at  the  upper  limit  of  the  atmosphere. 

4.  Kites  and  balloons :  Kites.  Exploration  of  the  free  air 
by  means  of  kites  and  balloons  may  be  said  to  have  assumed 
definite  proportions  at  the  beginning  of  the  twentieth  cen- 
tury. Numerous  attempts  to  use  kites  and  balloons  had 
been  made  prior  to  this  time,  and  with  a  large  measure  of 
success;  but  cooperative,  systematic  work  had  not  been 
carried  on.  Detailed  accounts  of  the  various  investigations 
may  be  found  in  a  report  presented  to  the  British  Association 
for  the  Advancement  of  Science,  at  the  Winnipeg  meeting 
in  1909;  also  in  an  address  delivered  by  Cave  before  the  Royal 

found  in  the  atmosphere  in  small  amounts,  the  weight  being  about  one  twentieth 
of  one  per  cent  and  the  volume  somewhat  less  than  this.  Hydrogen  is  found 
chiefly  in  the  higher  levels.  At  sea  level  the  amount  by  volume  amounts  to  about 
one  hundredth  of  one  per  cent. 

1  The  other  constituents  of  the  atmosphere  are  water  vapor  and  traces  of 
nitric  acid,  also  sulphuric  acid,  bacteria,  and  dust.  While  the  quantity  of  water 
vapor  is  small  and  varies  with  place  and  time,  it  is  perhaps  the  most  important 
of  all  the  substances  present  in  our  atmosphere  and 'most  affects  human  life. 
Its  functions  will  be  discussed  later  in  the  chapters  on  the  formation  of  the  clouds. 

Much  more  importance  is  now  attached  to  the  dust  content  of  the  air  than 
formerly.  Besides  serving  as  nuclei  for  condensation,  dust  plays  an  important 
role  in  absorbing  and  radiating  heat.  Volcanic  eruptions  are  now  known  to 
alter  materially  the  transparency  of  the  air.  For  example,  the  violent  eruption 
of  Mount  Katmai,  which  occurred  on  June  6,  7,  and  8,  1912,  resulted  in  notice- 
able dust  effects,  which  have  been  studied  in  their  relation  to  transparency  by 
Kimball  and  others;  and  with  regard  to  radiation  by  Abbot,  Fowle,  and  others. 
The  effect  of  the  dust  in  August  was  so  considerable  that  the  direct  radiation  of 
the  sun  was,  by  the  interposition  of  the  dust  cloud,  reduced  by _  about  20  per 
cent  at  the  stations  at  Bassour,  in  Algeria,  and  Mount  Wilson,  in  California. 
It  has  been  shown  that  not  only  the  Katmai  eruption,  but  also  other  great  erup- 
tions of  former  years,  have  materially  decreased  the  direct  radiation  of  the  sun 
and  apparently  altered  the  temperature  of  the  earth.  Further  discussion  will 
be  found  in  the  sections  treating  of  radiation. 


8  THE  PRINCIPLES  OF  AEROGRAPHY 

Meteorological  Society,  January  21,  1914.     From  these,  the 
following  abridged  history  is  largely  taken. 

So  far  as  is  known,  Wilson  of  Glasgow  was  the  first  to  use 
kites  for  scientific  purposes.  In  1749  he  raised  thermometers, 
by  means  of  several  kites;  and  on  one  occasion 
the  toP  kite  reached  an  amazing  height,  disap- 
pearing into  the  white  summer  clouds.  Three 
years  later  Franklin  made  his  well-known  experiments.  Kites 
with  thermometers  attached  were  again  used  in  1821  and  in 
Franklin  1836;  and  in  1847  attempts  were  made  at  the 

Kew  Observatory  to  measure  wind  velocity  and 
temperature  with  the  aid  of  kites.  In  1865  Nares  invented 
a  storm  kite  for  use  in  carrying  life  lines  ashore  in  case  of 
shipwreck.  Archibald,  in  1882,  tried  to  ascertain  the  increase 
of  wind  velocity  with  elevation,  and  obtained  differential 
measurements  to  a  height  of  300  meters.  He  also  introduced 
the  use  of  steel  wire  in  place  of  string,  and  in  1887  took  the 
first  photograph.1 

In  1885  observations  of  electric  potential  were  made  by 
McAdie  at  Blue  Hill  Observatory.  The  observations  were 
made  at  moderate  elevations  by  means  of  kites,  the  kite  string 
having  been  wound  with  fine  copper  wire.  Later,  kites  were 
used  by  Weber  in  Germany  for  the  same  purpose.  Eddy, 
in  1890,  devised  a  tailless  kite,  a  modification  of  the  Malay 
type,  and  raised  thermometers  to  moderate  heights.  He 
also  succeeded  in  obtaining  photographs  from  cameras  thus 
lifted.  But  the  most  important  advance  in  kite 
work  was  due  to  Lawrence  Hargrave  of  Sydney, 
Australia,  who  in  1893  or  thereabouts  invented 
the  box  form  of  kite.  Modifications  of  this  form  have  been 
used  by  meteorologists  in  all  countries. 

As  a  result  of  the  successful  use  of  kites  in  exploring  the 
upper  air,  and  a  report  by  A.  Lawrence  Rotch  submitted  to 
the  International  Meteorological  Conference  at  Paris  in 
September,  1896,  the  Conference  recommended  that  similar 
investigations  be  undertaken  by  the  various  official  weather 
services  of  the  world.  From  this  time  forth  the  use  of  the 

1  See  also  Blue  Hill  Observatory  Reports  for  1897;  Rotch,  Sounding  the  Ocean 
of  Air;  Quart.  Jour,  of  the  Royal  Met.  Soc.,  April,  1914;  and  Nature,  May  28,  1914. 


A  BRIEF  HISTORY  OF  AEROGRAPHY  9 

kite  spread  rapidly.     In  1898  daily  flights  were  attempted  at 
eighteen  stations  in  the  United  States.     Notwithstanding  the 
many  failures,  over  a  thousand  records  were  obtained,  a  few 
of   which   exceeded   2,000   meters.     In    1901    Rotch   demon- 
strated the  feasibility  of  using  kites  at  sea;  and 
in  the  summer  of   1892  Dines  flew  kites  of  his 
own    design    from    steamships.     Many    investi- 
gators have  perfected  the  details  of  kites  and  kite  meteoro- 
graphs.    Among    these    may    be    mentioned    Bell,    Koppen, 
de  Bort,   Berson,   Elias,   Hergesell,   Marvin,    Fergusson,    and 
Clayton.     Under  the  direction  of  de  Bort,  kites  were  flown 
day  and  night  whenever  possible  for  nine  months  in  1902-1903. 
The  maximum  height  reached  was  5,900  meters.      In  1905  de 
Bort  and  Rotch  organized  expeditions  for  studying,  by  means 
of    kite   records,    upper-air    conditions    over   the 
equatorial  Atlantic.     The  results  of  these  experi-     Greatest  alti- 
ments  will  be  given  later.     The  greatest  altitude    by  kites 
reached   by   kites  —  7,044   meters — was   attained 
at  Mount  Weather,   Virginia,   on   October  3,    1907.     During 
the  ascent  nine  kites  were  employed. 

Balloons.     The  first  use  made  of  the  balloon  for  meteoro- 
logical purposes  was  by  Dr.  John  Jeffries,  a  native  of  Boston 
and    graduate    of    Harvard,    living    in    England. 
On  November  30,  1784,  Jeffries  made  an  ascent 
from    London    with    a    French    aeronaut    named 
Blanchard,  paying  the  latter  one  hundred  guineas.     He  car- 
ried with  him  a  barometer,  a  thermometer,  a  hygrometer,  an 
electrometer,   a   mariner's   compass,    and   six   glass-stoppered 
bottles  filled  with  distilled  water.     The  bottles  were  emptied 
at  various  levels  and  sealed;  and  the  samples  of  air  thus  ob- 
tained were  later  analyzed  by  Cavendish.     The  rate  of  fall 
in   temperature   with   elevation   as   observed   by       jeffries 
Jeffries  (1°C.  for  every  200  meters)  and  the  de-       crosses  the 
crease  of  humidity   agree   fairly  well   with  later       channel 
determinations.     Incidentally,   Jeffries,   on  Janu- 
ary 7,  1785,  crossed  the  British  Channel  in  about  two  hours, 
starting  from  Dover  and  landing  in  the  forest  of  Guines  near 
the  spot  celebrated  in  history  as  the  "field  of  the  cloth  of 
gold." 


10  THE  PRINCIPLES  OF  XEROGRAPHY 

In  1803-1804  Robertson,  a  Belgian  physicist,  made  three 
ascents,  from  Hamburg  and  Petrograd,  the  last  for  the  Russian 
Academy,  with  the  express  purpose  of  determining  the  change 
in  the  rate  of  evaporation.  A  height  of  2,430  meters  was 
reached.  Biot  and  Gay-Lussac,  on  August  24,  1804,  under 
the  auspices  of  the  Academy  of  Sciences,  Paris,  made  an 
ascent,  reaching  an  altitude  of  4,000  meters;  and  on  Septem- 
ber 16,  Gay-Lussac,  at  an  elevation  of  7,400 
on  magne?ism  meters,  made  certain  experiments  on  magnetism 
and  also  collected  samples  of  air.  Two  note- 
worthy ascents  were  made  from  Paris  in  1850  by  Barral  and 
Bixio,  who  demonstrated  the  great  thickness  of  certain  clouds, 
and  showed  that  in  some  cases  this  exceeded  4,000  meters. 

In  1852  the  British  Association  for  the  Advancement  of 
Science  interested  itself  in  aerial  exploration,  and  four  ascents 
were  made  for  the  Kew  Observatory  by  John  Welsh.  The 
objects  were :  first,  to  determine  the  rate  of  fall  of  temperature 
with  elevation;  second,  to  note  the  change  in  humidity;  third, 
to  make  a  collection  of  air  samples  at  various  altitudes,  and 
finally  to  analyze  light  from  the  clouds.  The  thermometers 
were  inclosed  in  a  metal  tube  through  which  air  was  forced. 
This  was  the  first  use  of  an  aspiration  thermometer,  more 
perfect  forms  of  which  were  developed  later  by  Dr.  Assmann. 
Welsh  attained  heights  of  from  4  to  7  kilometers,  and  found 
that  the  temperature  fell  uniformly  until  at  a  certain  level, 
varying  with  the  day,  the  temperature  remaining 
practically  constant.  Glaisher  and  Coxwell,  from 
1862  to  1866,  made  twenty-eight  ascents  with  the 
view  to  determining  the  rate  of  temperature  fall  and  the  varia- 
tion in  humidity  and  electrical  potential.  In  cloudy  weather 
the  rate  of  fall  was  approximately  one  degree  Centigrade  for 
165  meters,  while  in  clear  weather  the  rate  was  only  half  of 
this.  At  a  height  of  8  kilometers  no  water  vapor  existed. 

On  September  5,  1862,  Glaisher  and  Coxwell  reached  a  height 
of  1 1,200  meters ;  but  as  Glaisher  was  unconscious  for  a  period  of 
about  thirteen  minutes,  and  the  observations  were  uncertain, 
the  actual  height  reached  is  a  matter  for  doubt.  On  April 
15,  1875,  Tissandier,  Spinelli,  and  Sivel,  acting  for  the  French 
Academy,  attained  an  elevation  of  8,530  meters;  and  although 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  11 

they  resorted  to  oxygen  inhalation,  the  companions  of  Tissandier 
were  asphyxiated,  while  he  himself  was  unconscious  for  some 
time.  On  December  4,  1894,  Dr.  A.  Berson  attained  a  height 
of  9,600  meters,  and  later  (1901)  Berson  and  Suring  reached  a 
known  height  of  10,500  meters  (and  probably  reached  10,800 
meters) ,  both  being  unconscious  at  the  maximum  height. 

In  order  to  compare  the  temperatures  obtained  by  Glaisher 
and  others  with  the  values  obtained  with  properly  ventilated 
and  protected  instruments,  simultaneous  ascents  were  made 
from.  London  and  Berlin  in  1898.  The  results  showed  that  the 
earlier  readings  of  thermometers  were  in  error  owing  to  faulty 
exposure.  Rotch,  in  ascents  made  in  Paris  and  Berlin,  demon- 
strated that  a  self-recording  thermometer  registered  eight 
degrees  higher  than  a  sling  thermometer,  and  the  latter  two 
degrees  higher  than  an  Assmann  aspirated  thermometer. 

In  1893  Hermit e  and  Besancon  put  into  practical  form  an 
idea  that  had  been  prevalent  for  some  time,  namely,  the  use 
of  small,  free  balloons  capable  of  lifting  light 
instruments.  A  varnished  paper  balloon  (IJ Aero-  balloon? 
phile)  filled  with  coal  gas  was  first  used,  but  later 
goldbeater's  skin  was  employed.  With  such  balloons,  which 
weighed  about  14  kilograms,  a  number  of  ascents  were  made 
between  1893  and  1898.  A  silk  balloon  (the  Cirrus,  capacity 
250  cubic  meters,  weight  42  kilograms)  made  eight  ascents 
from  Berlin  between  1894  and  1895,  carrying  instruments 
inclosed  in  an  aspirated  tube  designed  by  Assmann.  The 
greatest  elevation  reached  by  the  Cirrus  was  18,500  meters, 
where  a  temperature  of  206°  A.  was  recorded. 

In  1896  the  International  Meteorological  Conference  at 
Paris  appointed  a  committee  consisting  of  De  Fonvielle,  Her- 
mite,  Assmann,  Erk,  Hergesell,  Pomortzeff,  and 
Rotch  to  organize  a  series  of  simultaneous  inter-  organization 
national  ascents.  The  committee  also  considered 
the  question  of  uniformity  in  methods  of  observation  and  the 
interchange  of  instruments.  At  the  beginning  of  the  twentieth 
century  the  work  of  exploration  was  officially  under  way  in 
many  lands;  also  unofficially,  as  in  the  case  of  De  Bort  and 
Rotch.  Between  April,  1898,  and  1902,  the  former  sent  up 
no  fewer  than  258  ballons-sondes,  which  reached  heights  of 


12 


THE  PRINCIPLES  OF  A&ROGRAPHV 


11,000  meters.  Similar  apparatus  was  used  in  the  Atlantic 
expeditions  of  Rotch  and  De  Bort;  also  by  Hergesell,  in  1902. 
At  St.  Louis,  in  1905,  Rotch  made  the  first  series  of  registering 
balloon  ascents  in  America  (Figs.  2,  3,  and  4). 

At  the  second  meeting  of  the  International  Committee,  in 
1898,  it  was  resolved  that  (1)  thermometers  of  less  thermal 


FIG.  2.     FIRST  ASCENSION  IN  AMERICA,  ST.  Louis,  SEPTEMBER  15,  1904 

inertia  than  those  previously  used  were  necessary ;  (2)  efficient 
ventilation  was  indispensable ;  (3)  instruments  should  be  tested 
before  the  ascents,  under  circumstances  similar  to  those  encoun- 
tered during  the  ascents;  (4)  an  aspiration  psychrometer  sus- 
pended at  least  five  feet  below  the  car  was  the  only  instrument 
suitable  for  manned  ascents. 


A  BRIEF  HISTORY  OF  A&ROGRAPHY 


13 


The  International 
Commission  for  Scien- 
tific Aeronautics, 
under  the  leadership  of 
Hergesell,  has  been  the 
main  authority  in 
upper-air  investigation-. 
The  results  of  simulta- 
neous observations 
with  kites,  manned 
balloons,  free  balloons, 
and  sounding  balloons, 
and  also  those  made  at 
certain  mountain  sta- 
tions, have  been  pub- 
lished regularly.  These 
international  ascents 
began  November  14, 
1896,  with  France, 
Germany,  and  Russia 
participating.  Since 
then,  and  until  the  be- 
ginning of  the  European 
war  in  1914,  interna- 
tional ascents  were 
made  on  the  first  Thurs- 
day in  each  month;  on 
three  successive  days 
three  times  each  year; 
and  on  six  successive 
days  once  each  year. 

Congresses  have 
been  held  as  follows: 
at  Strassburg,  in  1898; 
at  Paris,  in  1900;  at 
Berlin,  in  1902;  at 
Petrograd,  in  1904;  at 
Milan,  in  1906;  at  Monaco,  in  1909;  and  at  Vienna,  in 
1912.  Further  meetings  were  prevented  by  the  European  war. 


FIG.  3.     LAUNCHING  BALLON-SONDE 


14 


THE  PRINCIPLES  OF  A&ROGRAPHY 


Many  observatories  in  various  parts  of  the  world  cooperate. 
The  latest  report  shows  the  following  list:  Trappes,  Uccle, 
Soesterberg,  Prinsenberg,  Pyrton  Hill,  Limerick, 
Manchester,  Bergen,  Christiania,  Copenhagen, 
Pa  via,  Monticalieri,  Milan,  Verona,  Ferrara, 
Modena,  Florence,  Livorno,  Vigna  di  Valle,  Monte  Cassino, 
Mileto,  Zurich,  Friedrichshafen,  Stuttgart,  Strassburg,  Aachen, 


FIG.  4.     FILLING  BALLON-SONDES  AT  ST.  Louis 

Cologne,  Hamburg,  Lindenberg,  Munich,  Vienna,  Pola, 
Trieste,  Pavlovsk,  Nijni-Oltchedaeff,  Sebastopol,  Tiflis,  Ekat- 
erinburg, Vladivostok,  Gizeh,  Batavia,  Blue  Hill,  and  Mount 
Weather.  Other  stations  recently  established  are  at  Simla 
under  Dr.  Walker,  at  Helwan  in  Egypt,  at  Tenerife,  and  a 
station  in  Uruguay.  Particularly  to  be  noted  is  the  station 
in  Spitsbergen,  where  German  observers  remain  not  only  in 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  15 

the  summer  but  also  through  the  winter,  to  study  the  atmos- 
phere in  the  Arctic  regions;  also  the  station  at  Batavia,  Java, 
where  Dr.  Van  Bemmelen  is  doing  such  excellent  work  on  the 
winds  in  the  upper  air  over  the  equatorial  regions  (Fig.  5) . 

"The  most  complete  observatory,"  says  Cave,  "for  upper- 
air  research  is  that  at  Lindenberg.  This  observatory  was 
founded  under  the  direct  personal  interest  of  the 

Kaiser;  and  under  the  direction  of  Dr.  Assmann      The     , 
1  •     1  -  £          1        --n,       Lindenberg 

has  carried  out  an  immense  amount  of  work  with      Observatory 

kites,  captive  balloons,  and  registering  balloons. 
Ascents  of  one  sort  or  another  are  made  every  day  in  the  year, 
and  on  the  international  days  a  large  number  of  ascents  are 
made  each  day.  The  Kaiser  has  also  shown  his  interest  in  the 
subject  by  giving  to  the  International  Commission  a  trans- 
portable observatory  that,  in  the  first  instance,  has  been 
erected  on  the  Peak  of  Tenerife,  where  the  Spanish  govern- 
ment now  proposes  to  build  a  permanent  observatory. 

' '  But  it  is  not  only  in  the  permanent  observatories  that  work 
is  being  done.  No  expedition  for  scientific  exploration  would 
to-day  be  complete  without  some  means  of  studying  the  upper 
air.  Dr.  Simpson  worked  with  balloons  in  the  Antarctic  in 
Captain  Scott's  expedition;  and  both  Captain  Amundsen  and 
the  Danish  Expedition  to  Greenland  propose  to  study  the 
upper  air. 

"Many  expeditions  have  been  dispatched  for  the  sole  pur- 
pose  of   aerological   research.     M.    Teisserenc   de   Bort   and 
Professor  Rotch  chartered  a  steamer,  which,  in 
the  years  1905,  1906,  and  1907,  traversed  various       ^™la°^ 
parts  of  the  eastern  Atlantic,  between  the  tern-       expeditions 
perate  zone  and  the  equator,  and  obtained  most 
interesting  results  from  their  observations.     The   Prince  of 
Monaco  made  several  cruises  in  his  yacht,  the  Princess  Alice, 
in  company  with  Professor  Hergesell,  notably  to  the  neighbor- 
hood of  the  Canaries  and  to  Spitsbergen. 

"The  Lindenberg  Observatory  organized  probably  the  best 
equipped  expedition.  This  was  sent  out  for  the  study  of  the 
upper  air  in  tropical  Africa.  Under  the  charge  of  Dr.  Berson, 
twenty-three  ascents  of  registering  balloons  were  made  from 
a  steamboat  on  the  Victoria  Nyanza  from  July  to  September, 


16 


THE  PRINCIPLES  OF  A&ROGRAPHV 


^DIRECTIONS  AND  VELOCITIES  OF  BALLOONS 

MAY   1906 


Small  circles  in  lines  indicate  times  of  Observa- 
tion  made  with  Tr&nsit, (usually  every  mipute). 

of  balloon  is  indicated  by  Urge 
circle  at  end  of  line,  ihe  ac 


FIG.  5.     COURSES  TAKEN  BY  BALLOONS  IN  MAY,  1906 

1908 ;  great  heights  were  reached,  and  valuable  results  obtained : 
much  work  was  also  done  with  kites  and  pilot  balloons.  In 
the  international  week  in  July  of  the  same  year  Professor 
Palazzo  made  some  ascents  with  registering  balloons  from 
an  Italian  cruiser  in  the  neighborhood  of  Zanzibar. 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  17 

"The  most  recent  aerological  expedition  is  one  organized 
by  P.  Y.  Alexander  to  study  the  upper  air  over  the  valley  of 
the  Amazon;  this,  too,  has  been  put  under  the  charge  of 
Dr.  Berson." 

Following  the  loss  of  the  steamship  Titanic  in  April,  1912, 
the  Scotia  was  sent  by  the  British  Board  of  Trade  in  1913  as  an 
ice  patrol  in  the  North  Atlantic  and  equipped  with  balloons 
and  kites.  This  work  is  now  carried  on  by  the  U.  S.  Coast 
Guard  vessels  Seneca  and  Tampa  (formerly  the  Miami}. 

Dr.    E.    Barkow,    of    the    German    Antarctic    Expedition, 
obtained  during  a  floedrift  journey  in  Weddell  Sea,  in  the 
course    of    a    period    covering    209    days,    many 
records    by    kites,    captive    balloons,    and    pilot         German 
balloons.     The  greatest  height  reached  was  17,200         expedition 
meters,  February,  1911.     Of  256  ascents,  123  were 
made  by  kites,  13  by  captive  balloons,  and  120  by  pilot  balloons. 
The  greatest  elevation  reached  by  kites  and  captive  balloons 
was  2,750  meters;  the  average,  1,079  meters.     The  pilot  bal- 
loons frequently  exceeded  10,000  meters,  but  the  average  height 
was  3,598  meters.     Jost  and  Stolberg,  1912-1913,  sent  up  at 
Godhavn,  on  the  west  coast  of  Greenland,  120  pilot  balloons, 
one  of  which  reached  a  height  of  39  kilometers  (24.2  miles). 

Unusually  high  flights  in  the  United  States  have  been 
those  at  Huron,  S.  D.,  on  September  1,  1910,  when  an 
elevation  of  30,468  meters  was  reached  and  a 
corresponding  temperature  of  232°  A.  A  mini-  greaTaltitudes 
mum  temperature  of  218°  A.  was  recorded  at  a 
height  of  15,182  meters.  The  balloon  fell  at  Castlewood,  105 
kilometers  east-northeast.  On  July  30,  1913,  at  Avalon, 
Cal.,  a  sounding  balloon  reached  an  elevation  of  32,643 
meters,  where  the  temperature  was  231°  A.  The  pressure  was 
as  low  as  10  kilobars.  The  lowest  temperature,  219° A., 
occurred  at  a  height  of  18,263  meters.  Blair,1  discussing  six 
ascents  in  which  the  balloon  reached  the  20-kilometer  level, 
says  that  the  greatest  height  reached  was  31.6  kilometers  on 
July  9,  1914;  and  at  this  height  the  easterly  wind  had  a  speed 
of  19  meters  per  second,  lowest  temperature  211°  A.  at  a  height 
of  16  kilometers. 

1  Monthly  Weather  Review,  April,  1916,  p.  187. 


18 


THE  PRINCIPLES  OF  AEROGRAPHY 


FIG.  6.     DINES'S  LIGHT-WEIGHT  METEOROGRAPH 

During  the  ascent  the  meteorograph  is  suspended 
inside  the  bamboo  frame  or  "spider." 


The  lowest  temperature  thus  far  recorded  in  any  country 
was  obtained  in  an  ascension  at  Batavia,  Java,  December  4, 
1913,  when,  at  a  height  of  17,000  meters,  the  beginning  of 

the  stratosphere  in 
equatorial  regions, 
the  temperature 
fell  to  181°  A. 
(-91.9°  C.;  or 
-133°  P.).  In  this 
ascent  the  balloon 
reached  a  height 
of  26,040  meters; 
but  above  17,000 
meters  the  tem- 
perature rose 
steadily. 

Abbot,  Aldrich, 
and  Kramer  of  the 
Astrophysical  Ob- 
servatory of  the  Smithsonian  Institution  have  recently  made 
experiments  with  balloon  pyrheliometers  with  a  view  to 
obtaining  measurements  of  solar  radiation  at  great  heights. 
A  radiation  record  was  obtained  at  about  14,000 
nieters,  and  the  results  indicated  that  the  value 
of  the  solar  constant  (1.93  calories)  previously 
obtained  would  remain  the  same.  In  July,  1914,  records 
were  obtained  at  Fort  Omaha  at  a  height  of  25  kilometers. 
Pilot  balloons  are  small  free  balloons  which  have  been  used 
with  great  success  in  studying  the  wind  currents  above  the 
surface  layers.  An  account  of  the  methods  of 
observing  and  the  working  up  of  the  observations 
to  give  the  trajectory  from  which  the  wind  velocity 
and  direction  at  different  heights  are  obtained  is  given  in  a 
later  chapter.  (Fig.  6.) 

Ten  pilot  balloons  were  sent  up  on  different  days  in  1909  at 
Blue  Hill  Observatory,  and  eleven  were  sent  up  in  1910.  The 
pilot-balloon  ascension  of  July  7,  1909,  was  the  first  of  its 
kind  in  the  United  States.  In  the  most  recent  Australian 
soundings  self-recording  theodolites  are  employed. 


Pilot 
balloons 


A  BRIEF  HISTORY  OF  AEROGRAPH  Y 


19 


The  extreme  elevations  that  have  been  reached  by  various 
means  are: 

by  kites,  7,044  meters,  at  Mount  Weather,  Virginia,  October  3, 1907; 
by  manned  balloons,  10,500  meters  (Berson),  July  31,  1901; 
by  sounding  balloons,  37,000  meters,  at  Pavia,  Italy,  1912  (?); 
by  pilot  balloons,  height  determined  by  theodolite,  39,000  meters, 

at  Godhavn,  1912-13; 
by  airplane,  7,950  meters  (G.  Guidi),  November  7,  1916. 

The  Meteorological  Service  of  Canada  sent  up  94  balloons 
from  Toronto  or  Woodstock  between  January,  1911,  and  May, 
1915.  Of  these  53  gave  records.  Most  of  the  balloons  traveled 
in  an  easterly  direction,  only  three  going  to  the  west.  The 
height  reached  was  lower  in  winter  than  in  summer.  The 
average  height  of  the  base  of  the  stratosphere  was  11.4  kilo- 
meters (7.1  miles)  in  winter;  and  13.4  kilometers  (8.3  miles) 
in  summer.  The  stratosphere  had  a  lower  temperature  in 
summer  (211°  A.)  than  in  winter  (214°  A.). 

5.  Determining 
the  height  of  the 
atmosphere:  By 
observations  of  me- 
teoric phenomena. 
Estimates  of  the 
height  of  the 
atmosphere  based 
upon  observations 
have  been  made  by 
many  astronomers 
at  different  places. 
The  paths  of  shoot- 
ing stars,  meteors, 
or  bolides  thus 
plotted  indicate  a 
possible  height  of 
from  150  to  200 
kilometers.  Meas- 
urements of  the 
auroral  arc  and 
streamers  generally 


FIG.  7.     PILOT  BALLOON  AS  USED  AT  BLUE  HILL 
Pilot  balloons  were  sent  up  on  ten  days  in  1909  at 

Blue  Hill  Observatory,  and  on  eleven  days  in   1910. 

The  pilot-balloon  ascension  of  July  7,  1909,  was  the 

first  of  its  kind  in  the  United  States. 


20  THE  PRINCIPLES  OF  AEROGRAPHY 

give  much  lower  elevations  and  are  not  reliable,  owing  to 
difficulties    of    identification    of   the   point   of   measurement. 
Perhaps   the   best   results   of   the   measurements 
Shooting  of  meteors  are  those  given  by  the  great  displays 

bolides  >rS>  of  December  24,  1873,  and  February  18,  1912. 
The  Astronomical  Society  of  Antwerp  established 
an  international  scientific  body  (Le  Bureau  Central  Mete- 
orique)  for  the  study  of  meteors,  star  showers,  bright  bolides, 
and  every  other  body  of  meteoric  form.  Nagcl  at  Jena  and 
Hoffmeister  at  Sonneberg  maintained  simultaneous  observa- 
tions for  this  purpose  during  1913.  There  is  also  an  American 
Meteor  Society  with  headquarters  at  the  McCormick  Obser- 
vatory, University  of  Virginia. 

The  twilight  arch  may  serve  as  a  means  of  determining  the 
upper  limit  of  the  atmosphere.  It  is  well  known  that  sunlight 
illuminates  the  upper  boundary  of  the  atmosphere 
f°r  some  time  after  the  sun  sets;  and  also  in  the 
morning  for  some  time  before  the  sun  rises.  The 
angle  of  the  twilight  arch,  as  it  is  called,  is  found  to  vary 
from  15.5°  to  18°.  We  can  readily  construct  a  right- 
angled  triangle  in  which  one  side  will  represent  the  earth's 
radius,  a  second  side  the  earth's  radius  plus  the  height  of  the 
atmosphere,  and  the  included  angle  one  half  of  the  twilight 
arch,  say  9°.  The  ratio  of  the  longer  to  the  shorter  side  is 
the  secant  of  the  inclosed  angle,  or  1.01245.  If  we  take  as 
the  earth's  radius  at  45°  a  value  6,367,575  meters,  we  have 

r-\-h  =  r  secant  9°, 

h  =  r  (secant  9°-l), 

h  =  6,367,575  (0.01245) -79,276  meters. 

The  height  of  the  atmosphere  according  to  this  method  is, 
then,  about  79  kilometers,  or  50  miles.  Owing  to  refraction,  or 
the  bending  of  the  rays  as  they  pass  through  layers  of  air  of 
different  densities,  a  large  correction  is  necessary,  and  the  value 
given  above  is  probably  too  large  by  as  much  as  25  per  cent.1 

An  approximate  method  of  determining  the  height  of  the 
atmosphere    would    be    by    means    of    cloud    measurements, 

1  For  tables  giving  the    duration    and   intensity   of   astronomical  and  civil 
twilight  see  H.  H.  Kimball  in  Monthly  Weather  Review,  Nov.,  1916. 


A  BRIEF  HISTORY  OF  A&ROGRAPHY  21 

although  the  value  obtained  for  the  highest  cloud  would  not 
necessarily  be  the  limiting  value  for  air,  but,  rather,  water 
vapor.  It  is  of  historic  interest  to  note  that  cloud  heights 
were  determined  trigonometrically  from  two  stations  as  early 
as  1644  by  two  priests  of  Bologna,  Riccioli  and  Grimaldi. 
These  were  probably  lower  clouds,  and  it  is  unlikely  that 
values  as  great  as  11  kilometers,  the  cirrus  level  in  temperate 
latitudes,  were  obtained.  In  the  tropics,  clouds  reach  some- 
what greater  elevations,  perhaps  as  high  as  17  kilometers. 

6.  Distribution  of  gases  in  the  atmosphere.     In  many  of  the 
earlier  treatises  on  meteorology  the  atmosphere  was  assumed 
to  be  homogeneous.     According  to  this  belief  there  would  be 
a  uniform  distribution  of  the  gases  according  to 
temperature,   pressure,   and   density.     It  is   now       T^e  atmos- 

11  j    -LI  1      •  Ii  r\  phere  not  a 

clearly  proved  that  such  is  not  the  case.  On  perfect  gas 
account  of  well-marked  circulations  and  con- 
tinuous departure  from  any  state  of  rest  there  can  be  no  such 
uniformity.  In  other  words,  the  law  of  Dalton  by  which  each 
gas  would  arrange  itself  independently  of  th.3  others  is  not 
strictly  applicable.  The  gas  constant  for  the  air  is  not  a 
constant.  It  varies,  as  we  shall  see  later,  owing  to  the  non- 
adiabatic  character  of  the  atmosphere.  It  is  not  correct  to 
treat  the  atmosphere  as  a  dry,  perfect  gas.  A  homogeneous 
atmosphere  would  extend  to  a  height  of  nearly 
8,000  meters.  If  the  atmosphere  consisted  of 
oxygen  only,  since  the  density  of  oxygen  is  some- 
what greater  than  that  of  air,  a  homogeneous  oxygen 
atmosphere  would  extend  upward  a  less  distance,  or  about 
7,200  meters.  Nitrogen,  being  slightly  less  dense,  would 
reach  a  height  of  8,200  meters.  Carbon  dioxide,  being 
denser,  would  not  exceed  5,200  meters.  Hydrogen  would 
extend  to  115,000  meters.  A  homogeneous  water- vapor 
atmosphere  would  have  its  upper  limit  at  a  height  of  12,847 
meters.  Homogeneous  atmospheres,  however,  do  not  exist 
in  nature. 

Humphreys  gives  the  following  summary1  of  our  knowledge 
of  the  distribution  of  the  gases  in  the  atmosphere: 

i  Bulletin  of  the  Mount  Weather  Observatory,  1909-1910,  Vol.  II,  p.  GO. 


22  THE  PRINCIPLES  OF  AEROGRAPHY 

"The  distribution  of  the  atmospheric  gases  has  several 
times  been  calculated1  according  to  one  or  another  assumption 
as  to  the  vertical  temperature  gradient,  as  to  the  relative 
proportions  of  the  several  gases  in  dry  atmosphere,  and  even 
as  to  what  gases  are  actually  present. 

"The  subject  is  of  general  interest  and  also,  in  some  par- 
ticulars, of  distinct  importance  to  meteorology. 
And  for  these  reasons  it  seemed  worth  while  to 
recalculate  this  distribution,  since  the  most  recent 
determinations  of  the  factors  upon  which  it  depends  differ 
materially  from  those  formerly  assumed. 

"Therefore  the  accompanying  table,  calculated  according 

to  Ferrel's  formula  for  latitude  45°,  and  graphically  represented 

in  Fig.  8,  is  based  on  the  following  assumptions 

percentages       which  correspond,  we  believe,  to  approximately 

average  conditions: 

"(1)  That  the  several  gases,  in  addition  to  water  vapor, 
present  to  an  appreciable  extent  in  the  atmosphere,  and  their 
volume  percentages  in  day  air  at  the  surface  of  the  earth, 
are,  as  Hann2  gives  them: 

Nitrogen 78.03  Hydrogen 0.01 

Oxygen 20.99  Neon 0.0015 

Argon 0.94  Helium 0.00015 

Carbon  dioxide 0.03 

"(2)  That  water  vapor  is  present  to  the  extent  of  1.2  per 
cent  of  the  total  gases  at  the  surface  of  the  earth,  and  that 
it  decreases  rapidly  with  increase  of  elevation,  to  an  imper- 
ceptible amount  at  or  below  the  level  of  10  kilometers. 

"(3)  That  the  surface  temperature  is  284°A. 

"(4)  That  the  temperature  decreases  uniformly,  at  the  rate 
of  six  degrees  per  kilometer,  from  the  surface  to  an  elevation 
of  11  kilometers,  where  it  is  218°A. 

' '  (5)  That  beyond  1 1  kilometers  above  sea  level  the  tem- 
perature remains  constant  at  2 18° A. 

iFerrel,   Recent  Advances  in  Meteorology,  p.  37    (1888);  Dewar,  Proceedings 
Royal  Institution,  London,  Vol.  XVII,  p.  223  (1902);  Hann,  Lehrbuch  der  Mete or- 
ie,p.  8  (1906). 

2 Lehrbuch  der  Meteorologie,  p.  5. 


ALTITUDE 

km.O         10 


20 


SO 


PRESSURE 


0054 


0060 


,0067 


0076 


0090 


0123« 


10 


20 


30        40        50        60        70 

VOLUME   PER  CENT. 


80 


90       100 


FIG.  8.     DISTRIBUTION  OF  GASES  IN  THE  ATMOSPHERE 


After  Humphreys 


"(6)  That  convection,  and  therefore  constant  volume  per- 
centage of  the  gases,  except  as  slightly  modified  by  the 
presence  of  water  vapor,  obtains  throughout  the  region  of 
temperature  changes,  that  is,  from  the  surface  up  to  the 
region  of  constant  temperature. 

23 


24  THE  PRINCIPLES  OF  A&ROGRAPHY 

11  (7)  That  in  the  region  of  constant  temperature,  or  that 
Convection  above  11  kilometers,  there  is  no  convection  and 
limits  that  in  this  region  the  gases  distribute  themselves 

acccrding  to  their  molecular  weights. 

"PHg.   9  must,  therefore,  be  understood  to  represent  both 
what  we  know  of  the  lower  atmosphere,  and  what  we  have 
reason  to  believe  true  of  the  upper.     The  one  part  shows 
what  we  are  certain  of;  the  other  indicates  what  to  look  for. 
"Probably  that  fact  in  regard  to  the  gases  of  the  atmos- 
phere most   surprising    to    the    average  person  is,   when  its 
immense  importance  is  considered,  the  relatively 
of^ateTvapor  sma^  amount  of  water  vapor — an  amount  which 
even    at    the   surface   of   the   earth   often   is   no 
greater  than  that   of  argon,    and  for  the  total   atmosphere 
scarcely  one  fourth  as  great." 

7.  Molecular  weights.  In  order  that  we  may  have  some 
idea  of  the  relative  weights  of  the  various  gases  met  within 
our  atmosphere,  the  following  table  of  molecular  weights  is 
given : 

Air 28.735 

Water  vapor 17.880 

Oxygen 31.760 

Carbon  monoxide 27.880 

Carbon  dioxide 43.760 

Hydrogen 2.000 

Nitrogen 28.020 

The  densities  are  as  follows:  that  of  air  is  1  at  temperature 
273°A.,  and  under  standard  pressure  the  relative  density  of 
hydrogen  is  0.0696;  of  aqueous  vapor,  0.6221 ;  of  helium,  0.137; 
of  carbon  dioxide,  1.529;  of  argon,  1.3775;  of  oxygen,  1.1053; 
of  nitrogen,  0.9673.  If  densities  are  desired  in  terms  of 
molecular  weight,  then 

P  =  m  P/KT 

In  this  formula  m  is  the  molecular  weight;  P,  the  pressure  of 
a  standard  atmosphere  or  1,000,000  dynes  per  square  centi- 
meter; K,  the  radiation  energy  or  approximately  8,000,000 
dynes  per  square  centimeter;  and  7,  the  temperature  in  degrees 
absolute.  For  air  the  value  is  .0013;  water  vapor,  .0008; 
oxygen,  .0014;  carbon  monoxide,  .00125;  carbon  dioxide, 
.00197;  hydrogen,  .00009;  and  nitrogen,  .00126. 


CHAPTER  II 

UNITS  AND  SYMBOLS 

It  is  not  an  easy  matter  to  discard  terms  with  which  we 
have  long  been  familiar;  and  it  will  be  something  of  a  hard- 
ship for  the  present  generation  to  abandon  the  use  of  known 
terms  and  recognized  units,  and  adopt  others  in  their  place. 
The  change  to  the  new  notation,  however,  must  be  made  in 
order  to  meet  the  requirements  of  the  new  meteorology  now 
generally  called  "aerography."  The  adoption  of  the  new 
units  will  result  in  fewer  mistakes,  will  simplify  greatly  the 
compilation  of  data,  and,  above  all,  will  lead  to  definite  and 
precise  conceptions  of  the  phenomena  of  the  atmosphere, 
particularly  the  various  transformations  of  energy  which  are 
manifested  in  general  and  local  disturbances. 

8.  The  centimeter-gram-second  system.  This  system,  com- 
monly called  the  c.g.s.  system,  is  most  suitable  at  the  present 
time  and  is,  moreover,  international  in  character.  It  is  fun- 
damentally the  creation  of  Weber  (1852),  following  Gauss 
(1832).  The  first  unit,  that  of  length,  is  the  centi- 
meter,  or  hundredth  part  of  a  meter.  The  meter  is 
generally  defined  as  the  ten-millionth  of  the  meridian  passing 
through  Paris.  In  1795  the  French  Republic  made  this  the 
legal  standard  of  length,  and  an  arc  of  the  meridian  extend- 
ing from  Dunkirk  to  Barcelona  was  measured  by  Delambre 
and  Mechain.  The  standard  actually  is  the  distance  between 
two  graduations  on  a  platinum-iridium  bar  at  a  tempera- 
ture of  273°A.  (0°C.;  32°F.).  The  bar  is  preserved  in  the 
national  archives  of  France. 

The  second  unit,  that  of  quantity  of  matter,  is  the  gram. 
It  is  the  thousandth  part  of  the  quantity  of  matter  in  a 
standard  piece  of  platinum-iridium  called  the  Gram 

kilogram  prototype.     The  kilogram,  or  standard 
of  mass,  is  made    as    nearly  as   possible  equal  to  the  mass 
of  a  cubic  decimeter  of  distilled  water  at  maximum  density, 
277°A,  or  1,014  on  the  New  Absolute  scale. 

25 


26  THE  PRINCIPLES  OF  AEROGRAPHY 

The  third  unit  is  that  of  time  and  is  the  mean  solar  second; 
that  is  to  say,  there  are  86,400  such  seconds  in  a  mean  solar 
day.  These  units  are  sometimes  called  absolute, 
though,  strictly  speaking,  they  are  not.  Derived 
from  the  three  units  already  denned  are  others :  the  unit  of 
velocity  or  the  ratio  of  length  to  time,  the  conversion  factor 
being  lit.  The  unit  is  one  centimeter  per  second.  Another 
is  the  unit  of  momentum,  or  quantity  of  motion.  This  is 
the  mass  multiplied  by  the  velocity.  The  conversion  factor  is 
milt.  The  unit  of  acceleration  is  one  centimeter  per  second 
per  second,  and  the  conversion  factor  ///2.  The  unit  of  area 
is  /2,  the  unit  of  volume  Z3,  and  the  unit  of  density  or  ratio  of 
mass  to  volume  m/l3. 

The  unit  most  frequently  used  is  that  of  force,  which  in 
the  c.g.s.  system  is  called  the  dyne,  or  the  force  which  will 

impart  to  the  unit  mass  (a  gram)  an  acceleration 
The  dyne  .  .  .,  1  T-V 

of  one  centimeter  per  second  per  second.     J^orce 

is  measured  by  the  rate  of  change  of  momentum,  and  the 
conversion  factor  is  ml/t2.  Forces  are  generally  measured  by 
weights  or  the  earth's  attraction  upon  given  masses.  The 
attraction  varies  from  place  to  place  and  with  distance  from 
the  center  of  the  earth.  If  we  designate  by  g  the  acceleration 
due  to  gravity,  we  can  say  that  the  weight  of  a  gram  at  any 
given  place  is  g  dynes.  In  the  old  English  system,  when  mass 
was  expressed  in  pounds,  length  in  feet,  and  time  in  seconds, 
the  unit  of  force  was  called  the  poundaL  In  the  new  system, 
since  a  pound  is  453.6  grams  and  a  foot  is  30.48  centimeters,  a 
poundal  would  be  13,825  dynes. 

Work  done  by  a  force  may  result  in  change  of  velocity  or 
change  of  form,  the  former  being  a  change  in  kinetic  energy 
The  er  an<^  ^e  latter  a  change  in  potential  energy.  The 

unit  of  work  is  the  erg,  and  the  conversion  factor 
is  mP/t2.  To  raise  one  kilogram  ten  meters  requires  one 
million  ergs.  The  unit  of  power  is  the  watt,  or  ten  million 

ergs  per  second.  In  the  old  system  the  unit  was 
The  watt  „__  ,  ,  Tr  .  ,-,  • 

a  horse-power,  or   550   foot-pounds.     It    in   this 

system  the  acceleration  of  gravity  be  32.2  feet  per  second  per 
second  and  the  foot  be  30.48  centimeters,  then  17,710x30.48 
X  13, 825  will  give  the  equivalent  value  in  the  new  system; 


UNITS  AND  SYMBOLS  27 

and  this  divided  by  ten  million  gives  for  a  horse-power  746.4 
watts.  A  kilowatt  therefore  is  1,000/746  horse-power  (1.34 
horse-power) . 

In  1911  the  American  Institute  of  Electrical  Engineers 
adopted  746  watts  as  the  exact  value  of  a  horse-power.  It  is 
the  rate  of  work  expressed  by  550  foot-pounds  per  second  at 
50°  latitude  and  at  sea  level.  The  continental  horse-power 
is  736  watts,  or  75  kilogrammeters  per  second  at  Berlin,  52° 
N.  latitude. 

In  the  metric  system  the  Greek  prefix  deka  indicates  ten; 
hecto  (which  is  seldom  used),  a  hundred;  kilo,  a  thousand; 
and  mega,  a  million.  The  Latin  prefixes  are  deci,  indicating 
one  tenth;  centi,  one  hundredth;  milli,  one  thousandth;  and 
micro,  one  millionth,  in  this  case  a  Greek  prefix. 

The  meter  was  originally  intended  for  use  as  a  geographic 
unit.  Instead  of  90  degrees,  the  earth's  quadrant,  as  previously 
indicated,  was  divided  into  one  hundred  grades.  A  grade 
in  turn  was  divided  into  a  hundred  minutes,  and  a  minute 
into  a  hundred  seconds.  The  meter  is  one  tenth  Decimali- 
of  the  centesimal  second,  or  one  ten-millionth  zation 
of  the  earth's  quadrant.  The  decimalization  of  of  angles 
angles  has  not,  however,  been  generally  adopted,  though  the 
system  has  certain  advantages.  Meanwhile,  the  arc  of  90 
degrees  on  the  earth's  surface  is  approximately  10,000  kilo- 
meters; and  one  degree  of  arc,  or  60  nautical  miles,  is 
equal  to  111.1  kilometers.  A  nautical  mile  is  equal  to  69 
statute  miles. 

9.  The  unit  of  pressure.  Unfortunately,  in  meteorology 
atmospheric  pressure  has  been  represented  in  three  different 
ways:  first,  in  units  of  height  of  a  column  of  Various 

mercury  in  vacuo, —  as  760  millimeters,  or  29.92  pressure 

inches;  second,  in  units  of  weight, —  as  in  speak- 
ing of  the  equivalent  weight  of  the  atmosphere  (1,033.3  grams 
per  square  centimeter,  or  10,333  kilograms  per  square  meter, 
or  14.66  pounds  per  square  inch,  or  2,111.2  pounds  per  square 
foot) ;  and  finally,  in  units  of  force. 

Forces  are  usually  measured  by  weights,  or  the  earth's 
attraction  upon  given  masses.  This  attraction  varies  from 
place  to  place  with  distance  from  the  center  of  the  earth. 


THE  PRINCIPLES  OF  ARROGRAPHV 


If  we  designate  by  g  the  acceleration  due  to  gravity,1  then 
the  weight  of  a  gram  at  any  given  point  is  g  dynes.  The 
value  of  g  is  given  by  the  formula  of  Bowie, 

g0  =  978.039  (1  +  0.005294  sin2</>- 0.000007  sin2  20) 
in  which  g  is  the  value  at  sea  level  and  cf>  the  latitude.2  It 
is  now  proposed  to  measure  atmospheric  pressure  in  units 
of  force.  It  has  been  suggested  that  the  term 
"Newton"  be  used  as  the  unit  of  force  100,000 
dynes,  and  the  term  "Pascal"  for  the  absolute 
atmosphere,  or  1,000,000  dynes.  The  unit  of  force,  then, 


"Newton 
"Pascal" 


NORMAL  ACCELERATION  OF  GRAVITY  AT  SEA  LEVEL  IN 
CENTIMETERS  PER  SEC.  PER  SEC. 
</>=  latitude  of  place 


0             !        0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

pole        083  21 

80° 

983  .  06 

.09 

.12 

.14 

.16 

.18 

.19 

.20 

.21 

.21 

70° 

982.61 

.66 

.72 

.77 

.82 

.87 

.91 

.95 

.99 

3.03 

60° 

981.91 

.99 

2.07 

.14 

.22 

.29 

.35 

.42 

.49 

.55 

50° 

981.07 

.16 

.24 

.33 

.42 

.50 

.59 

.67 

.76 

.84 

40° 

980.17 

.26 

.35 

.44 

.53 

980.62 

.71 

.80 

.89 

.98 

30° 

979.32 

.40 

.48 

.56 

.65 

.73 

.82 

.90 

.99 

0.08 

20° 

978.64 

.69 

.76 

.82 

.89 

.95 

9.02 

.10 

.17 

.25 

10° 

978.19 

.22 

.25 

.29 

.33 

.38 

.42 

.47 

.52 

.58 

Equator 

978  .  03 

.03 

.04 

.05 

.06 

.07 

.09 

.11 

.13 

.16 

is  the  dyne;  and  the  unit  of  acceleration,  the  gal.3  A  dyne 
is  approximately  1/980  of  the  weight  of  a  gram. 

Actual  measurements  of  the  intensity  of  gravity  have  been 

1  " Gravity"  is  the  term  used  for  the  phenomenon   of  weight,   or  of  the 
acceleration  of  a  body  falling  to  the  earth;  and  at  any  place  it  is  the  resultant 
of  the  earth's  attractive  force,  "gravitation,"  and  the  centrifugal  force  due  to 
the  earth's  rotation.     This  distinction  between  "gravity"  and  "gravitation"  is 
made  by  the  U.  S.  Coast  Survey. 

2  More  generally  expressed  in  a  formula  of  Helmert : 

go  =  9.80617  (1  -.002644  cos  2^>  +  0.000007  cos2 20) 

=  9.8062  meters  at  sea  level  and  latitude  45°. 

The  value  980.665  was  adopted  in  1888  by  the  International  Committee  on 
Weights  and  Measures  and  has  since  been  continued  for  convenience  although 
it  is  a  conventional  standard  and  not  exactly  equal  to  the  value  at  45°.  The 
best  value  of  the  normal  acceleration  is  980.624. 

3  Whipple  has  proposed  the  term  "leo"  (the  last  syllable  of  Galileo)  for  the 
unit  of  acceleration;  but  —  as  Klotz  points  out  in   Nature,  August   13,   1914, 
p.  611  —  Weichert  used  the  term  "gal"  (first  syllable  of  Galileo)  for  this  unit  in 
connection  with  earthquake  motion  as  early  as  1909,  and  it  has  come  into  use  in 
seismology.     The  "milligal"  is  approximately  one  one-millionth  of  g. 


UNITS  AND  SYMBOLS  29 

made  at  many  points  on  the  earth's  surface  and  these  are 
expressed  in  accelerations.  Normal  values,  as  they  are  called, 
are  calculated  by  the  general  formulas  of  geodesy.  The  inten- 
sity of  gravity  decreases  from  the  pole  to  the  equator;  the 
variation  may  be  readily  obtained  from  the  table  on  p.  28. 
Barometer  readings  are  frequently  reduced  to  standard  gravity 
by  applying  the  correction  for  latitude,  which  is  small,  and  a 
further  correction  for  elevation. 

NEW  UNITS 

For  many  years  meteorologists  felt  the  need  of  a  unit 
of  pressure  that  had  some  definite  relation  to  standard  units. 
Early  in  1908  McAdie  suggested  several  modifica- 
tions of  the  units  and  symbols  then  employed.  The  new 
In  the  Monthly  Weather  Review  for  August,  1908,  in  pressure 
a  plea  for  new  units  generally,  he  described  an 
original  method  of  representing  pressure  variations  in  percent- 
ages or  permillages  of  a  standard  pressure.  In  an  extended 
discussion  of  the  paper  in  the  Monthly  Weather  Review  for 
March,  1909,  Koppen  suggested  that  instead  of  the  sea-level 
pressure,  a  new  base — namely,  the  pressure  represented  by  the 
value  1,000,000  dynes— be  used.  The  new  pressure  base  is 
the  pressure  at  a  height  of  106  meters  (348  feet)  above  sea 
level.  In  other  words,  instead  of  using  the  pressure  indi- 
cated by  760  millimeters  (29.92  inches),  which  in  force  units 
would  be  1,013,307  dynes,  obtained  by  multiplying  1,033.291 
grams  per  square  centimeter  by  the  normal  acceleration  of 
gravity  980.66,  we  might  use  the  force  corresponding  to  a 
pressure  reading  of  750  millimeters  (29.53  inches).  In  April, 
1909,  Koppen  presented  to  the  Aerological  Congress  at  Monaco 
a  strong  plea  for  the  use  of  the  new  units  of  pressure. 

The  first  use  of  the  new  units  in  the  United  States  was 
during  1910,  at  San  Francisco.     In  Europe,  under  the  stimu- 
lation of  Shaw,  Koppen,  and  other  prominent  meteorologists, 
there    has  been  a  ready  acceptance  of  the  new 
units.     The  use  of  the  centibar  and  the  millibar     andmUlibar" 
has  become  general,  and  daily  and  weekly  weather 
reports  are  published  with  isobars  in  millibars  and  temper- 
atures in  the  Absolute  scale  (273° A.,  32°F.).     On  January  1, 


30 


THE  PRINCIPLES  OF  AEROGRAPHY 


1914,  the  new  units  were  used  at  Blue  Hill  Observatory,  and  on 
the  same  date  the  United  States  Weather  Bureau  began  the 
use  of  these  units  in  a  daily  map  of  the  northern  hemisphere. 
Bjerknes  in  his  Dynamic  Meteorology  and  Hydrography 
used  the  term  ' '  bar  "  as  a  short  and  convenient  term  for 
the  megadyne  atmosphere.  Unfortunately  European  mete- 
orologists were  not  aware  that  this  word  had  already 
been  defined  and  accepted  in  scientific  usage,  although  not 
The  chan  e  verv  Senera^y  adopted.  It  is  of  some  impor- 
fromC  tance  that  a  short  term  should  be  used  for  the 

Sloba? t0  basic  unit,  and  therefore  the  better  usage  is  to 
make  the  bar  represent  the  force  of  one  dyne  per 
square  centimeter  instead  of  a  million  dynes.  Thus  the  milli- 
bar becomes  the  kilobar.  The  following  table  contrasts  the 
two  systems,  the  New,  or  American  system,  and  the  Old, 
or  European  system. 

AMERICAN  AND  EUROPEAN  SYSTEMS 


NEW 

OLD 

Chemists  and  physi- 
cists (to  be  univer- 
sally used  hereafter) 

Former  aerolo- 
gists   (to 
be  abandoned) 

Remarks 

1  megabar  

1  bar  ...  

The  absolute  atmosphere;  equal  to 

• 
1  kilobar  . 

1  millibar 

750.1  mm.  mercury,  or  .987  usual 
sea-level  atmosphere.     One  mega- 
dyne  per  square  centimeter  acting 
through  one  cubic  centimeter  does 
one  megerg  of  work. 
One  kilodyne  per  square  centimeter. 

1  bar  

1  microbar.  .  . 

One    dvne    per    square    centimeter 

acting  through  one  cubic  centimeter 
does  one  erg  of  work. 

For  conversion  of  inches  and  fractional  parts  into  kilobars  see  Table  7. 

There  would  be  no  objection  to  giving  the  term  megabar 
or  absolute  atmosphere  some  convenient  nickname,  such  as 
"aer,"  if  megabar  is  too  ponderous.  It  has  been  suggested 
by  Professor  Richards  that  for  historical  reasons  the  pressure 
of  10,000,000  dynes  (ten  absolute  atmospheres)  might  be 
named  after  some  pioneer  in  meteorology,  as  von  Guericke  or 
Torricelli,  after  the  analogy  of  the  "watt,"  "joule,"  "ampere," 
etc. ;  but  this  need  not  be  insisted  on  at  present. 


UNITS  AND  SYMBOLS  31 

10.  International  symbols.  The  use  of  certain  symbols  was 
agreed  upon  by  the  Congress  at  Vienna  in  1873  and  these 
have  come  into  widespread  use.  These  symbols1  are: 

0  Rain  V  Frostwork  (rough)  forming  T  Distant  thunder 

•Y-  Snow          co  ICG  coating  (smooth)  forming  oo  Haze 

^  Hail  _£_>  Drifting  snow  0  Solar  halo 

/\  Sleet  <_  Floating  ice  crystals  ®  Solar  corona 

=  Fog  __JILJ  Gale  q?  Lunar  halo 

_^_  Dew  j-^  Thunder  storm  vi;  Lunar  corona 

i i  Hoar  frost    </  Distant  lightning  ^^  Rainbow 

H  Surrounding  country  more  than  half  s^v  Aurora 
under  snow 

The  intensity  of  a  phenomenon  is  denoted  by  an  exponent, 

0  indicating  slight,   2,   great,   and  an  absence  of  exponent, 
moderate  intensity. 

The  time  of  occurrence  is  expressed  in  hours  and  tenths; 
morning  and  afternoon  are  indicated  by  A.  and  p.,  respectively; 
midnight  and  noon  by  12  p.  and  12  M.,  respectively,  the  hours 
being  counted  from  0  to  12,  commencing  at  midnight.  The 
continuance  of  a  phenomenon  is  indicated  by  a  dash  ( — ). 

Maximum  and  minimum  values  are  denoted  by  heavy- 
faced  type,  except  for  relative  humidity,  in  which  case  only 
the  minima  are  so  indicated. 

The  working  meteorologist  uses,  in  addition  to  the  symbols 
and  abbreviations  given  above,  certain  tables  for  dividing  by 
31  and  29;  also  keeps  conveniently  near  such  data  as  the 
number  of  hours  in  a  year,  8,760  (strictly  8,766  and  in  a  leap 
year  8,784).  Certain  astronomical  and  geodetic  data  are 
also  useful.  These  are  as  follows: 

GENERAL  GEODETIC  DATA 

1  cm.  equals  .3937  inch,  or  .0328  feet.     (1  mm.  is  roughly  .04  inch.) 
1  cm.2  equals  .155  square  inches. 

1  cm.3  equals  .061  cubic  inches. 

1  cm.  per  second  equals  .0224  miles  per  hour,  or  .0328  feet  per  second. 

The  equatorial  radius  of  the  earth  is  6,378,388  ±  18  meters,  or  3,963 

miles. 
The  polar  semi-diameter  of  the  earth  is  6,356,909  meters,  or  3,950 

miles. 

1  Professor  C.  F.  Talman  in  Monthly  Weather  Review,  May,  1916,  p.  265,  dis- 
cusses in  detail  the  various  meteorological  symbols  used  throughout  the  world. 


32  THE  PRINCIPLES  OF  A&ROGRAPHY 

The  reciprocal  of  flattening  is  1/29.4. 

The  circumference  of  the  equator  is  40,076,000  meters,  or  24,902 

miles. 
The  perimeter  of  the  meridian  ellipse  is  40,008,600  meters,  or  24,860 

miles. 
The  area  of  the  earth's  surface  is  196,940,000  square  miles,  or  510r 

044,000  square  kilometers. 
The  area  of  the  ocean  is  approximately  three  quarters  that  of  the 

whole  earth's  surface. 

The  mass  of  the  earth  is  5,984  X  1024  kilograms,  or  6  X  1021  tons. 
Themassof  the  atmosphere  is  5,263  X  1015  kilograms,  or  5.8  X  1015tons. 
The  mass  of  the  oceans  is  about  1.3  X  1024  kilograms,  or  1.3  X  1018tons. 
The  volume  of  the  atmosphere  is  approximately  4,080  X  1015  cubic 

meters;  and  since  a  cubic  meter  of  dry  air  weighs  1.293  kg.  the 

approximate  weight  of  the  atmosphere  is  5,263  X  1015  kgs.     This 

is  1/1,125,000  of  the  mass  of  the  earth. 
The  mean  density  of  the  earth  is  5.52. 
The  mean  density  of  the  surface  is  2.67. 
The  mean  density  of  the  ocean  is  1.03. 

A  mean  solar  day  is  24  hours,  3  minutes,  56  seconds  sidereal  time. 
A  sidereal  day  has  86,164  seconds,  or  23  hours,  56  minutes,  4  seconds 

mean  solar  time. 

A  sidereal  year  has  365.26  mean  solar  days. 
The  mean  distance  from  earth  to  sun  is  149,500,000  km.,  or  92,900,000 

miles. 

Solar  parallax,  8.796";  lunar  parallax,  3,422.68". 
Sun's  diameter,  1,392,000  km.,  or  865,000  miles. 
The  mean  distance  from  earth  to  moon  is  384,399  km.,  or  238,854 

miles,  or  60.3  terrestrial  radii. 

The  velocity  of  light  is  299,870  km.  per  second  (186,300  miles). 
The  time  required  for  light  to  traverse  the  mean  radius   of  the 

earth's  orbit  is  498.8  seconds. 

USAGE  OF  CERTAIN  LETTERS 

<j)  =  latitude  R  =  gas  constant 

X  =  longitude  V  (or  v)  =  volume 

p  =  density  y  and  v  used  also  in  some  equa- 
k  =  ratio  of  specific  heats  tions  for  velocity 

7  or  g  =  gravity  b=bar 

co  =  angular  velocity  of  the  earth  kb  =  kilobar 
6  =  temperature  /    "  , 

7r  =  3.14159265  m/s  =  meters  per  second 

P  and  B  =  barometric  pressure  e  =  base  of  natural  logarithms 
T  =  temperature  on  Absolute  scale  or  2.718281828 

No  uniformity  exists  in  the  usage  of  letters.  Unfortunately,  too,  letters  like 
g,  m,  p,  and  v  are  used  with  different  meanings.  An  international  standard 
of  notation  is  much  needed 


CHAPTER  III 

TEMPERATURE  SCALES 

ii.  The  nature   of  heat.     In   his  book,    The  Constitution 
of  Matter,  Professor  Ames  says: 

"There  is  no  word  in  our  language,  I  think,  which  is  so 
much  used  to  conceal  ignorance  as  'heat,'  and  no  word  about 
which  there  is  so  much  confusion  of  ideas  as  'temperature.' 
.  When  we  investigate  the  physical  differences 
between  a  hot  body  and  a  cold  one,  or  when  we  learn  by 
what  physical  processes  we  can  make  a  body  hotter,  we 
find  that  its  temperature  is  determined  by  the  average 
kinetic  energy  of  translation  of  the  molecules  of  the  body, 
neglecting  any  regular  systematic  motion.  (Thus  in  a  tuning 
fork  or  vibrating  string,  these  molecular  motions  are  not 
included  in  the  kinetic  energy  which  determines 
temperature.)  In  the  case  of  a  gas  inclosed  in 
a  cylinder,  if  we  push  the  piston  in,  there  will 
be  an  increased  kinetic  energy  of  the  molecules  which  will 
appeal  to  our  senses  as  a  rise  in  temperature.  Similarly, 
if  the  gas  expands,  pushing  out  the  piston,  it  does  work,  and 
the  average  kinetic  energy  of  the  molecules  decreases;  and 
we  all  know  that  when  a  gas  expands  its  temperature  falls, 
for  example,  as  shown  in  the  formation  of  clouds. 

"Thus  if  by   any   process   the   average   kinetic  energy   of 
translation  is  varied,  so  is  the  temperature. 

"Lord    Kelvin    many   years    ago    called    attention   to   the 
fact  that  it  was  possible  to  define  temperature  in  a  way  that 
would    be    entirely   independent    of    the    thermometric    sub- 
stance  used  in   the   measuring   instrument.     This   he   called 
the     'absolute'    temperature    system;    and    also      The 
showed  that  for  all  practical  purposes  this  system      "absolute" 
agreed  with  the  system  of  using  for  a  thermometer      temperature 
a    bulb    containing    hydrogen    or    nitrogen    and 
measuring   the    change  in   volume,   the  pressure  being  kept 
constant   as  the  temperature   changed.     He  further  proved 

4  33 


34  THE  PRINCIPLES  OF  ARROGRAPHY 

that  with  matter  there  is  a  definite  minimum  temperature, 
lower  than  which  it  is  impossible  to  reduce  a  body.  This  is 
called  'absolute  zero';  and  for  convenience  this  is  taken  as 
the  starting  point  of  the  absolute  scale." 

12.  The  measurement  of  heat.  Heat  may  be  measured 
in  dynamical  units  or  in  thermal  units.  In  the  former  the 
dimensions  are  the  same  as  those  of  energy,  and  the  con- 
version factor  is  ml*/t2.  In  other  words,  the  temperature  of 
a  body  may  be  considered  to  be  the  average  kinetic  energy 
of  its  molecules  and  be  designated  by  mass  multiplied  by 
velocity  squared.  When  measured  in  thermal  units  we 
determine  the  amount  of  heat  required  to  raise  unit  mass 
of  water  one  degree.  Here  the  conversion  factor  is  m6\ 
.  and  the  heat  unit  is  the  gram-calorie,  or  the 
quantity  of  heat  which  will  raise  the  tempera- 
ture of  a  gram  of  pure  water  one  degree.  Since  the  specific 
heat  of  water  varies  slightly  at  different  temperatures,  the 
value  of  the  gram-calorie  is  properly  one  one-hundredth  of 
the  total  heat  required  to  raise  the  temperature  of  a  gram 
of  water  from  273°A.  to  373°A.  Some  physicists  limit  the 
small  calorie,  or  therm,  to  the  quantity  of  heat 
which  will  raise  the  temperature  of  a  gram  of 
water  from  273°A.  to  274°A.  This  unit  is  called 
the  small  calorie  (or  therm)  because  engineers  find  a  larger 
mass  of  water  more  convenient  for  a  unit,  and  so  use  a  kilo- 
gram. The  large  unit  is,  then,  one  thousand  times  greater 
than  the  small  calorie. 

The  relative  amount  of  heat  required  to  raise  unit  mass 
of  any  substance  one  degree  compared  with  water  is  the 
specific  heat.  This  varies  according  as  the  pressure  and 
volume  remain  constant.  The  specific  heat  of 
Specific  heat  air  at  constant  pressure  is  0.24,  and  at  constant 
volume  0.17.  The  specific  heat  of  water  vapor 
at  constant  pressure  is  0.47,  or  nearly  twice  that  of  air.  The 
specific  heat  of  water  vapor  at  constant  volume  is  0.36.  In 
other  words,  it  requires  0.24  gram-calorie  to  raise  the  tem- 
perature of  a  gram  of  air  from  273°  A.  to  274°  A.,  if  the 
pressure  remains  constant,  and  twice  that  amount  for  a  gram 
of  water  vapor  under  constant  pressure.  The  ratio  of  the 


TEMPERATURE  SCALES 


35 


specific  heat  of  air  at  constant  pressure  to  the  specific  heat 
at  constant  volume  is  0.2375/0.1683,  or  1.41.  This  value, 
which  is  of  importance  in  aerography,  is  perhaps  best  rep- 
resented by  the  symbol  k,  although  in  many  treatises  the 
symbol  y  has  been  used.  The  ratio  of  the  specific  heats  of 
water  vapor  is  0.4734/0.3631,  or  1.30,  and  this  can  best  be 
designated  by  the  symbol  kr 

13.  Temperature  scales.     It   is   now   desirable   to   record 
temperatures  on  the  absolute  Centigrade  scale,  the  zero  being 
that  of  the  hydrogen  gas  thermometer,  or  practi-        Absolute 
cally  273.02°  below  the  zero  of  the  Centigrade        Centigrade 
scale  thermometer,  and  459.4°  below  the  zero  of 
the  Fahrenheit  scale,  which,  serviceable  for  many  years,  has 
now  outlived  all  usefulness,   and  is  practically  discarded  in 
scientific  work.     It  may  be  recalled  that  originally  this  scale 


McAdie  in  "Scientific  American" 

FIG.  9.     CONVENIENT  CONVERSION  SCALE 
This  scale  gives  conversion  of  temperatures  from  Fahrenheit  to  Absolute 

ran  from  -90°,  obtained  by  a  mixture  of  salt  and  ice,  to  +90°, 
the  temperature  of  the  human  body,  the  whole  making  180°. 
Later,  Fahrenheit  made  the  zero  of  the  scale  the  lowest  tem- 
perature of  the  winter  of  1709  at  Danzig,  32°  the  temperature 
of  melting  ice,  and  212°  the  temperature  of  boiling  water 
under  constant  pressure.  The  other  scales,  Reaumur's,  the 
Celsius,  and  Linnaeus'  modification  of  the  Celsius  (the  modern 
Centigrade),  make  the  freezing  and  boiling  temperatures  of 
water  fiducial  points.  The  Reaumur  scale  was  devised  by 
a  French  physicist  in  1731;  and  the  Celsius  by  a  Swedish 
astronomer  in  1742.  He  made  the  boiling  point  zero  and  the 
freezing  point  100. 


36  THE  PRINCIPLES  OF  A&ROGRAPHY 

The  new  zero  is  that  of  the  hydrogen-gas  thermometer. 
Here  the  temperature  is  directly  proportionate  to  the  pres- 
sure.    There  is  a  temperature  scale  known  as  the 
thermometer-8    absolute    energetic    or    thermodynamic    scale,    in 
which  the  ratio  of  any  two  temperatures  is  equal 
to  the  ratio  of  the  heat  absorbed  at  the  one  temperature  to 
the  heat  evolved  at  the  other  when  the  heat  is  transferred 
by    any    reversible    cyclical    process    whatever. 
dynamic"  scale   Unlike  gas  or  mercury  thermometer  scales,   the 
definition  of  temperature  does  not  here  involve 
a    relation    to    any    property    of    any    definite    substance. 
Temperatures  expressed  in  this  scale  are  proportional  to  the 
pressures  given  by  a  constant-volume  thermometer  filled  with 
a  perfect  gas.     There  is  a  slight  difference  between  the  two 
scales,  but  no  great  error  in  making  the  zero  273°  Centigrade 
below  the  temperature  of  freezing  water.     The  thermodynamic 
temperature  of  the  ice  point  is  273.1°  A.     For  rapid  conversion 
of  the  different  scales  see  Table  9.'     Or  in  a  general  way  the 
illustration  Fig.  9  may  be  used. 

A  modification  of  the  Absolute  scale  has  been  proposed  by 
the  author;  in  which  starting  from  the  absolute  zero,  marked 
0,  the  scale  divisions  have  a  value  of  .366  of  the  Centigrade 
or  Absolute  scale  division.  This  makes  the  reading  for  the 
temperature  of  melting  ice  1000.  No  degree  signs  are  used 
in  the  New  Absolute  scale,  as  these  are  to  be  reserved  for 
angular  measurement.  The  boiling  point  is  1366. 


CHAPTER   IV 

THERMODYNAMICS  OF  THE  ATMOSPHERE 

14.  The  specific  heat  of  air.  The  quantity  k,  the  ratio 
of  the  specific  heat  of  air  at  constant  pressure  to  the  specific 
heat  at  constant  volume,  is  of  much  importance  in  connection 
with  the  cooling  and  heating  of  the  free  air.  It  also  enters 
into  the  determination  of  the  velocity  of  sound  in  air.  Newton 
gave  a  formula  for  this  velocity,  based  upon  the  elasticity  of 
the  air;  and  he  considered  that  the  square  root  of  pressure 
divided  by  density  would  give  the  velocity  of  The  velocity 
propagation  of  sound.  But  compression  follows  of  propaga- 
raref action  rapidly  in  the  transmission  of  sound  tlon  of  sound 
waves.  Laplace  pointed  out  that  during  such  rapid  changes 
of  pressure,  the  pressure  did  not  rise  proportionately  to  the 
density,  as  Boyle's  law  would  require.  Instead  of  pressures 
being  inversely  proportional  to  the  volumes,  that  is,  p/pi  =  Vi/v, 
they  must  be  inversely  proportional  to  the  volumes  raised 
to  a  certain  power,  which  was  k;  or  p/p\  =  (vjv)k.  Since 
VP  =  VIPI,  the  velocity  is  equal  to  the  square  root  of  kp/d. 
The  ratio  p/p  (pressure  to  density)  is  proportional  to  the 
absolute  temperature,  and  therefore  the  velocity  is  directly 
proportional  to  the  square  root  of  the  absolute  temperature. 

At  273°  A.  the  velocity  of  sound  in  air  is  33,176  centimeters 


k-i 


per  second.  The  value  of  -j-  is  0.286,  and  this  is  equal 
to  the  temperature  gradient  in  air  multiplied  Adiabatic 
by  the  gas  constant  R.  The  adiabatic  tempera-  temperature 
ture  gradient  for  dry  air  is  9.847°  per  thousand  gra  ien 
meters.  The  value  is  the  same  for  moist  air,  provided  no 
condensation  occurs;  otherwise  the  heat  of  condensation 
partly  compensates  for  adiabatic  cooling.  The  average 
value  for  saturated  air  in  the  lower  strata  is  not  far  from 
6  degrees  per  thousand  meters,  or  approximately  half  the 
adiabatic  rate.  Dines,  in  discussing  the  vertical  distribution 
of  temperature,  has  shown  that  under  actual  conditions  the 
adiabatic  gradient  holds  only  for  small  changes  of  height  and 

37 


38 


THE  PRINCIPLES  OF  A&ROGRAPHY 


not  for  large  changes,  and  therefore  pressure  in  the  higher 
strata  is  greater  than  that  given  by  the  formula. l  The  differ- 
ences for  different  heights  under  average  conditions  are: 


1  km. 

2km. 

3  km. 

4km. 

5  km. 

6  km. 

7  km. 

8  km. 

.3°A. 

.6 

1.0 

1.8 

2.6 

3.5 

4.5 

6.0 

9  km. 

10km. 

11  km. 

12km. 

13  km. 

14km. 

15km. 

7.6 

9.7 

11.3 

13.9 

16.7 

19.8 

25.0 

Thus,  if  a  balloon  containing  air  could  rise  with  sufficient 
rapidity  to  15  kilometers,  allowing  the  gas  inside  to  expand 
adiabatically,  the  fall  of  temperature  would  be  only  125°, 
instead  of  the  150°  given  by  the  commonly  used  formula. 

The  difference  between  the  specific  heat  of  air  at  constant 
pressure  and  constant  volume  is  0.0692;  and  this  is  generally 
represented  by  the  symbols  CP-CV. 2  This  is  equal  to  the 
Mechanical  heat  equivalent  of  work  multiplied  by  the  gas 
equivalent  constant.  The  mechanical  equivalent  of  heat 
(sometimes  called  the  work  equivalent  of  heat) 
(/)  is  41,840,000,  or  one  calorie  equals  4.184  X  107ergs,  or 
4.184  joules  when  not  done  against  gravity.  This  would 
raise  a  gram  of  air  against  gravity  426.8  meters. 

Joule's  equivalent,  as  it  is  often  called,  is  connected  with 
the  quantity  of  heat  by  the  equation  ML2T~2,  which  is  equal 
to  JH  or  JM6.  The  conversion  factor  is  Pt~-6~l.  When 
heat  is  measured  in  dynamical  units,  /  is  a 
number.  Emissivity  is  the  quantity  of  heat 
given  off  by  a  substance  per  unit  time  per  unit  of  surface  per 
unit  difference  of  temperature  between  the  surface  and  the 
surrounding  medium.  The  conversion  factor  is  ml~2t~l.  In 
thermometric  units,  by  substituting  P  for  m  the  factor  becomes 
It'1,  and  in  dynamical  units,  mt~*6~l. 

Latent  heat  is  the  ratio  of  the  number  representing  the 

T   .  quantity  of  heat   required   to  change   the   state 

Latent  heat        M,  y  :*•  . 

of  a  substance  to  the  number  representing  the 

quantity  of  matter  in  the  substance.     The  conversion  factor 

1  Dines,  Quart.  Jour,  of  the  Royal  Met.  Soc.,  July,  1913,  p.  187. 

2  See  section  16  (p.  41),  on  dynamical  heating  and  cooling  of  the  air. 


Emissivity 


THERMODYNAMICS  OF  THE  ATMOSPHERE          39 

is  simply  the  ratio  of  the  temperature  units,  or  6.  In  dynam- 
ical units  the  factor  is  /2r2.  When  a  solid  is  at  its  melting 
point,  or  a  liquid  at  its  boiling  point,  no  change  of  tempera- 
ture is  noticeable  when  heat  is  added.  There  is,  however,  a 
change  in  the  internal  energy.  Conversely,  when  substances 
return  to  their  initial  condition,  as  when  water  vapor  con- 
denses, an  equivalent  quantity  of  heat  is  set  free  or  does 
work  of  another  kind.  The  heat  absorbed  or  set  free  in  such 
changes  of  state,  occurring  with  no  apparent  change  in  tem- 
perature, is  called  the  "latent  heat."  Thus  we 
have  the  latent  heat  of  fusion  and  of  vaporization 
(or  sublimation).  The  latent  heat  of  fusion  of 
ice,  as  recently  determined,  is  79.63  calories.  To  melt  a  gram 
of  ice,  then,  would  require  theoretically  80  calories.  It  should 
be  remembered,  however,  that  a  gram  of  ice  is  by  weight 
a  little  more  than  a  cubic  centimeter;  and  if  pure  ice  is  used 
only  73  calories  are  needed.  To  melt  ice  con- 
taining a  percentage  of  solids  such  as  ice  from  J^*®1^ 
sea  water,  a  still  smaller  number  of  calories  will  vaporization 
suffice.  The  latent  heat  of  vaporization  is  536 
calories;  that  is,  it  requires  that  much  heat  to  change  a  gram 
of  water  into  vapor  at  the  boiling  point  (373° A.). 

15.  The  equation  of  elasticity.  According  to  the  law  of 
Boyle  and  Mariotte,  pv  =  p0v0,  in  which  v  represents  the 
volume  of  unit  mass  of  a  gas  at  a  pressure  p  and  temperature 
273°A.  In  other  words,  if  the  temperature  is  v  1  me 
constant  and  the  pressure  be  doubled,  the  volume  inversely 
will  decrease  one  half;  and  conversely,  if  the 
volume  be  increased  twofold,  the  pressure  de- 
creases one  half.  The  law  is  not  true  for  great  or  small 
pressures,  but  it  does  hold  for  the  atmosphere  when  the 
pressure  is  about  870  kilobars.  At  very  low  pressures  the 
air  seems  to  lose  its  elasticity. 

The  law  of  Charles  and  Gay-Lussac  introduces  the  tem- 
perature as  a  factor  in  controlling  volume  and  Tem  erature 
pressure.  It  rests  upon  the  experimental  fact  effect  on 
that  the  volume  of  a  gas  increases  1/273  of  its 
volume  at  273°A.  for  each  degree  increase  of 
temperature;  and  it  decreases  at  the  same  rate.  Thus  the 


40 


THE  PRINCIPLES  OF  A&ROGRAPHY 


laws  given  above  may  be  combined  into  the  following  formula : 

/w  =  A00(l  + 1/2730), 

which  is  another  way  of  saying  that  at  a  temperature  of  0°A., 
according  to  the  kinetic  theory  of  gases,  there  would  be  no 
velocity  of  molecular  motion.  If,  then,  we  let  6  represent 
temperature  on  the  absolute  scale  we  may  write  the  equation 
pv  =  R6,  which  means  that  if  the  volume  remains  constant 
the  pressure  is  proportional  to  the  absolute  temperature,  and 
conversely,  the  volume  is  proportional  to  the  absolute  tem- 
perature if  the  pressure  remains  constant.  As  the  elastic 
force  or  pressure  is  determined  by  the  mean  square  of  the 
velocities  of  the  molecules,  it  follows  that  the  mean  is  as  the 
absolute  temperature. 

VALUE  OF  R,  GAS  CONSTANT,  IN  CHARACTERISTIC 
EQUATION  FOR  AIR 


0 

A. 

Saturation  pressure 

7? 

in  kilobars 

250 

0.79 

2871 

260 

2.34                                     2872 

270 

4.90                                      2875 

273 

6.11 

2876 

275 

7.05 

2877 

280 

9.99 

2880 

285 

13.96 

2884 

290 

19.30 

2890 

295 

26.16 

2895 

300 

35.41 

2905 

305 

47.17 

2915 

310 

62.27 

2930 

The  characteristic  equation  is,  then,  pv  =  Rd  or  p  =  Rpd, 
in  which  R  is  the  gas  constant  equal  to  2,870  when  the  pres- 
The  sure  is  given  in  kilobars  or  e.g. s  units;  6  equals 

characteristic     temperature  in   degrees   Absolute;   and  p  equals 
equation  the  density  or  ^     Different  values  of  the  gas 

constant  R  have  been  given  by   Shaw  computed   from  the 
formula 

RW  PW 


R, 


8fJo 


where  R0  is  the  value  for  dry  air;   Rw  for  mixture  of  air  and 


THERMODYNAMICS  OF  THE  ATMOSPHERE         41 

water  vapor  which  is  saturated  at  temperature  6  and  has  a 
partial  pressure  of  dry  air  of  1,000  kilobars  at  the  freezing 
point. 

The  characteristic  equation  is  true  for  an  ideal  gas  only. 
Another  law,  due  to  Amedeo  Avogadro,  states  that  under  the 
same    conditions    of    temperature    and    pressure 
equal  volumes  of  gases  contain  the  same  number 
of  molecules.     The  number  of  molecules  in  one 
cubic  centimeter  of  any  gas  at  a  pressure  of  1,000,000  dynes 
and  a  temperature  of  273°  A.  is  27  billion  billion. 

1 6.  Dynamical  heating  and  cooling  of  the  air.  When  air 
or  any  other  gas  —  or,  even  a  solid  —  is  compressed,  there 
is  an  increase  of  temperature  which  is  generally  apparent  if 
the  compression  is  sudden.  Thus  with  a  hand-compression 

pump:    when    the    piston    is    forced    down,    the 

-,        -•  r    .  -,  -,  ,  -,     .      .,  .        .  Compression 

molecules  of  the  gas  have  their  kinetic  energy     increases 

increased;  and  conversely,  when  the  gas  or  air  kinetic 
is  allowed  to  expand,  the  kinetic  energy  of  the 
molecules  is  decreased,  and  energy  has  been  expended  in 
pushing  back  the  piston.  In  the  former  case  the  tempera- 
ture rises;  in  the  latter  the  temperature  falls,  and  it  appears 
that  the  average  kinetic  energy  is  proportional  to  the  abso- 
lute temperature.  The  fact  that  "the  average  kinetic  energy 
of  the  molecules  is  equal  to  a  constant  multiplied  by  the 
absolute  temperature  is,"  says  Ames  in  his  Constitution  of 
Matter,  "believed  to  be  also  true  for  solids  and  liquids,  the 
constant  being  the  same  as  for  a  gas." 

When  a  body  falls  rapidly,  or  slowly,  or  in  any  way  moves 
from  a  higher  to  a  lower  level,  work  is  done  and  resistance 

overcome.     Potential    energy    due    to    elevation 

i_        i_  1  i    •          1  •        •  -i       Kinetic 

has  been  changed  into  kinetic  energy  expended      energy 

in    giving    momentum     to     the    mass.     If    the      transformed 

.   *?  £•>'•<  111  j.1  •  mt°  heat 

resistance  or  friction  be  marked,  then  the  rise 
in  temperature  is  generally  noticeable.  Conversely,  in  raising 
or  lifting  a  body,  work  is  done  against  the  force  of  gravity, 
and  potential  energy  is  acquired  at  the  expense  of  kinetic 
energy,  or  heat.  Thus  when  a  mass  of  air  rises,  as  it  will  when 
heated,  or  falls,  as  it  will  when  cooled,  work  is  performed; 
and  energy  of  either  potential  or  kinetic  kind  is  transformed 


42  THE  PRINCIPLES  OF  ARROGRAPHY 

into  heat.  But  the  sum  total  of  the  energy  remains  the  same 
according  to  the  principle  of  the  conservation  of  energy. 

The  lifting  of  one  kilogram  of  dry  air  against  the  force  of 
gravity  through  a  distance  of  one  meter  has  been  taken  as 
the  unit  of  work,  and  is  known  as  the  kilogram-meter.  In 
the  e.g. s.  system  the  unit  would  be  more  properly  one  gram 
lifted  a  distance  of  one  centimeter,  or  the  erg.  If  we  take  the 
dyne  as  the  unit  of  force,  then  one  dyne  per  square  centimeter 
acting  through  one  cubic  centimeter  does  one  erg  of  work. 

A  million  of  the  small  units  of  work  is  called  a  megerg, 
although  some  writers  prefer  the  term  "megalerg."  This 

_,  causes    some    confusion;    likewise    the   fact    that 

The  megerg  •,/••••  -, 

there   are  two   units  of  heat  in  general  use:  a 

large  one  much  used  by  engineers,  which  is  the  amount  of 
heat  required  to  raise  the  temperature  of  a  kilogram  of  water 
one  degree;  and  the  small  unit  or  therm,  which  will  raise  the 
temperature  of  a  gram  of  pure  water  one  degree.  Since  there 
The  is  a  slight  difference  in  the  specific  heat  of  water 

mechanical  at  different  temperatures  the  small  calorie,  or 
equivalent  therm,  is  more  precisely  defined  as  the  amount 
of  heat  that  will  raise  the  temperature  of  a  gram 
of  pure  water  from  273°A.  to  274°A.  Now,  it  has  been  shown 
experimentally  that  the  energy  which  can  raise  the  tempera- 
ture of  a  gram  of  water  one  degree  could  do  the  mechanical 
work  of  lifting,  against  the  force  of  gravity,  one  gram  of 
water  42,683  centimeters.  This  is  called  the  mechanical 
equivalent  of  heat  under  standard  gravity;  that  is,  at  45°  lati- 
tude and  at  sea  level.  This  is  usually  expressed  I/A  Con- 
versely there  is  a  heat  equivalent  of  work  usually  expressed  by 
the  symbol  A.  Its  value  is  0.00002343. 

When  heat  is  added  to  a  given  volume  of  air,  say  one  cubic 
centimeter,  and  this  is  free  to  expand  in  one  direction,  some 
of  the  heat  goes  toward  increasing  the  temperature  and  some 
is  expended  in  the  work  of  expansion.  Expressing  it  more 
directly  in  percentages,  we  may  say  that  of  a  given  quantity 
of  heat,  71  per  cent  is  utilized  in  raising  the  temperature  and 
29  per  cent  is  spent  in  expanding  the  air  against  atmospheric 
pressure.  This  ratio  is  determined  as  follows:  To  raise  the 
temperature  of  a  gram  of  pure  water  one  degree  needs  one 


THERMODYNAMICS  OF  THE  ATMOSPHERE         43 

small  calorie;  to  raise  the  temperature  of  a  gram  of  dry  air, 
0.2375  calorie,  which  is  the  specific  heat  of  air  at  constant 
pressure.  This  is  sometimes  written 

CP  =  Cv  +  AR. 

Now,  a  cubic  centimeter  of  pure,  dry  air  weighs  somewhat 
more  than  a  thousandth  of  a  gram,  or  0.00129305  gram;  and  if 
we  multiply  this  by  the  specific  heat  of  air  we  have  0.000307, 
which  is  heat  that  would  raise  the  temperature  one  degree 
and  expand  the  air  1/273  of  its  volume.  The 
expansion,  however,  has  been  done  against  the  ^expansion 
pressure  of  a  sea-level  atmosphere,  or  1,033,300 
gram-centimeters.  If  we  use  the  new  atmosphere,  we  have 
1,000,000  dynes  divided  by  273,  or  3,663  dynes.  This  is 
accomplished  by  0.307  unit  of  heat,  and  therefore  a  whole  unit 
would  do  11,931  dynes.  To  this  we  must  add  the  work  done  in 
rising  from  sea  level  to  the  new  base,  which  is  106  meters,  or 
348  feet,  above  sea  level.  There  would  also  be  a  slight  change 
in  gravity.  The  total  would  be  12,330  dynes.  But  the  work 
or  mechanical  equivalent  of  one  unit  of  heat  is  42,683  gram- 
centimeters,  and  the  ratio  of  this  to  the  former  is  as  1  to  0.29. 

When  there  is  no  expansion  of  air  and  no  work  is  done, 
the  whole  amount  of  heat  is  consumed  in  raising  the  tem- 
perature. In  this  case  the  increase  in  temperature  is  to  the 
increase  when  work  is  performed  as  1  to  0.71,  or  as  1.41  to  1. 
In  the  same  way,  the  heat  that  will  raise  the  temperature  of 
a  cubic  centimeter  of  dry  air  without  change  in  volume  is  to 
the  heat  required  when  there  is  change  in  volume  as  1  to  1.41. 

The  specific  heat  of  air  under  constant  pressure  being 
0.237,  the  specific  heat  under  constant  volume  is  0.237/1.41,  or 
0.169.  The  difference  between  the  specific  heats  is  0.068. 

It  follows  from  what  precedes  that  where  no  heat  is  added 
to  or  subtracted  from  a  mass  of  air,  and  it  expands  or  con- 
tracts under  varying  pressure,  such  work  must  be  done  at 
the  expense  of  its  own  heat.  Hence  the  heat  loss  (when  air 
expands  by  coming  under  diminished  pressure  adiabatically 
or  without  the  addition  of  heat  from  other  source)  for  every 
1/273  part  of  volume  increase  is  0.29  of  a  calorie.  To  cool 
a  whole  degree  the  expansion  would  have  to  amount  to  1/79 


44 


THE  PRINCIPLES  OF  AEROGRAPHY 


of  the  volume.  We  have  seen  that  the  height  of  the  homo- 
geneous atmosphere  is  7991  meters;  hence  7991/79  gives  us 
approximately  the  height  101.2  meters,  to  which  air  must  be 
lifted  to  cool  one  degree,  or  practically  one  degree  per  hundred 
meters.  Strictly,  the  adiabatic  rate  is  9.8  per  kilometer,  but 
this  value  is  true  only  for  small  changes. 

The  air  is  not  dry,  but,  on  the  contrary,  especially  in  the 
lower  levels,  is  often  saturated,  and  so  a  strictly  adiabatic 

condition  is  rarely  met.  The  specific  heat  under 
condition  rare  cons"tant  pressure  is  not  a  constant  but  a  variable 

quantity.  Circulation  and  radiation  materially 
alter  the  heat  distribution.  In  problems  of  dynamic  meteor- 
ology, observations  which  contain  the  height  z,  the  pressure 
p,  and  temperature  6,  but  omit  the  velocity  of  circulation 
and  the  direction  of  motion  q,  are  defective;  and  if,  further- 
more, the  amount  of  water  vapor  present  and  the  various 
changes  of  state  are  not  considered,  then  there  can  be  no 
precise  data  regarding  heat  distribution.1 

ACTUAL   TEMPERATURE    GRADIENTS 

The  following  tables,  given  by  Dines,2  give  the  fall  of  tem- 
perature per  kilometer,  or  the  approximate  temperature 
gradient,  for  every  month: 


km. 

0-1 

1-2 

2-3 

3-4 

4-5 

5-6 

6-7 

7-8 

Jan  

5°  A. 

4 

4 

5 

7 

7 

6 

7 

Feb. 

5°  A. 

5 

4 

G 

7 

6 

7 

t 

March  .  .  . 

4°  A. 

6 

4 

G 

7 

6 

7 

7 

April  

6°A. 

0 

5 

6 

7 

6 

7 

7 

May  

6°  A. 

G 

5 

6 

G 

7 

7 

6 

June  

C°A. 

0 

5 

6 

G 

7 

7 

7 

July  

6°  A. 

5 

5 

G 

G 

6 

8 

6 

Aug  

6°  A. 

4 

5 

G 

6 

/ 

7 

7 

Sept  

5°  A. 

3 

5 

6 

6 

7 

7 

6 

Oct  

4°  A. 

5 

5 

6 

6 

7 

6 

7 

Nov  

5°  A. 

3 

5 

6 

6 

6 

8 

6 

Dec. 

5°  A. 

3 

5 

6 

6 

7 

7 

6 

Average.  . 

5.3 

4.8 

4.8 

6.0 

6.3 

6.6 

7.0 

6.6 

LF.  H.  Bigelow,  "Thermodynamics  of  the  Earth's  Non-adiabatic  System," 
Am.  Jour,  of  Sci.,  Dec.,  1912.  Also  same  author's  "Treatise  on  Circulation 
and  Radiation  in  the  Atmospheres  of  the  Earth  and  of  the  Sun,"  1915. 

Trans,  of  the  Royal  Soc.  of  London,  Series  A,  Vol.  211,  pp.  253-278. 


THERMODYNAMICS  OF  THE  ATMOSPHERE 


45 


km. 

8-9 

9-10 

10-11 

11-12 

12-13 

13-14 

Mean 

0-9 

Tan.  . 

6 

4 

3 

o 

1 

o 

5  8 

Feb 

6 

3 

3 

-1 

1 

o 

5  9 

March  
April  
May 

6 
6 

7 

4 

4 
5 

3 
3 
4 

-2 

i 

-1 

0 

-1 

-1 

0 
0 

o 

5.9 
6.2 
6  2 

June  

7 

6 

4 

-1 

-1 

0 

6  3 

July  

Aue.  . 

7 
8 

8 

7 

4 
4 

0 
1 

-1 
o 

1 

o 

6.1 
6  2 

Sept 

8 

/ 

5 

1 

1 

o 

5  9 

Oct  
Nov.. 

7 
7 

7 
5 

4 
4 

1 
1 

1 
1 

1 
1 

5.9 
5  8 

Dec  
Average  

7 
6.8 

4 
5.3 

3 
3.5 

1 
-0.1 

1 

0.2 

1 

0.3 

5.8 
6.1 

Thus  it  is  evident  that  the  gradient  in  the  free     Gradient  per 

,  ,  -.,.          .  .  ,     «o  thousand 

air  under  usual  conditions  is  approximately  6   per     meters 

thousand  meters,   while  under  adiabatic  conditions   the   fall 
is  9.8°. 

NOTE. — The  student  will  find  detailed  free-air  data  in  Supplement  No.  3  of 
the  Monthly  Weather  Review  issued  Dec.  1,  1916. 


CHAPTER  V 

STRATOSPHERE  AND  TROPOSPHERE 

17.  The  stratosphere  and  troposphere.  The  most  impor- 
tant outcome  of  the  numerous  soundings  of  the  upper  air  has 
G  ,.  been  the  discovery  of  a  cessation  of  fall  in  tern- 

ceases  at  a  perature  at  a  certain  height.  Above  this  level 
^e  temperature  remains  stationary  or  even  rises, 
and  it  has  been  found  that  the  height  at  which 
the  gradient  ceases  or  reverses  varies  with  season  and  latitude. 

The  actual  cessation  of  the  fall  in  temperature  was  first 
noticed  by  Teisserenc  de  Bort  in  June,  1899,  and  confirmed  in 
March,  1902.  The  phenomenon  was  discussed  in  May,  1902, 
by  Assmann,  who  made  special  experiments  to  prove  that 
the  condition  was  not  the  result  of  defective  ventilation  of 
the  thermometers  employed.  The  absence  of  a  vertical  de- 
crease of  temperature  led  to  the  use  of  the  name  isothermal 
layer,  or  upper  inversion;  but  de  Bort  soon 
inversion1"  introduced  the  terms  stratosphere  for  the  upper 
layer,  and  troposphere  for  the  lower  levels  where 
convection  did  occur.  Gold  suggested  the  term  "advective" 
for  the  upper  region;  because  any  interchange  of  air  would 
occur  chiefly  through  horizontal  motion,  while  in  the  lower 
the  interchange  would  take  place  through  ascensional  and 
descensional  currents,  and  therefore  might  well  be  called 
Major  and  "convective."  These  names,  however,  have  not 
minor  come  into  general  use.  There  are  frequent  in- 

mversions  versions  of  temperature  near  the  earth's  surface 
and  it  has  been  suggested  by  the  author  that  these  be  called 
the  minor  inversions,  while  the  change  at  the  stratosphere  be 
called  the  major  inversion. 

The   average   height   at   which   the   change   occurred   was 

Variation  of       found  by  Teisserenc  de  Bort  to  be  11  kilometers; 

average  and  he  also  called  attention  to  the  fact  that  the 

eig  *  height  varied  in  highs  and  lows,   averaging   12.5 

kilometers  in  the  former  and  10  kilometers  in  the  latter.     At  a 

46 


STRATOSPHERE  AND  TROPOSPHERE 


47 


level  of  10  kilometers  the  difference  in  pressure  between  an 
average  high  and  an  average  low  would  be  approximately 
10  kilobars,  while  the  difference  between  the  pressures  at 
the  level  of  the  stratosphere  in  ordinary  highs  and  lows  is 
about  70  kilobars. 

Numerous  records  of  soundings  have  been  made  public 
through  the  International  Commission  for  Scientific  Aero- 
nautics (see  p.  13).  A  good  illustration  of  a 
successful  sounding  is  given  in  abridged  form 
below.  The  ascent  was  made  at  the  observatory 
at  Uccle,  Belgium,  June  9,  1911,  during  pleasant 
weather.  A  height  of  31,780  dynamic  meters,  or  32,430 
meters,  was. reached.  A  temperature  of  212°A.  was  recorded 


Record  of 
soundings 
made  at 
Uccle 


Time 

Pressure 

Elevation 
Met. 

Temperature 
Abs. 

Gradient 
A  //1  00m. 

R.  H. 

kb. 

mm. 

7:00 

1,001 

751 

100 

290° 

81 

7:05(?) 

900 

675 

1,000 

287 

.... 

.  . 

797 

598 

2,000 

281 

.056 

.... 

705 

529 

3,000 

275 

.066 

51 

7:18 

621 

466 

4,000 

269 

.042 

34 

7:20(?) 

547 

410 

5,000 

264 

.089 

479 

359 

6,000 

251 

.086 

30 

7:34 

313 

235 

9,000 

234 

.084 

30 

7:38 

271 

203 

10,900 

222 

.065 

7:44 

199 

149 

12,000 

212 

.039 

29 

7:47:04! 

168 

126 

13,040 

213 

.021 

29 

7:48 

160 

120 

13,340 

213 

-.007 

29 

7:52 

129 

97 

14,650 

218 

-.031 

30 

7:56 

103 

78 

16,050 

223 

-.07 

29 

8:02 

72 

54 

18,370 

218 

.01 

29 

8:12 

36 

27 

22,720 

222 

.00 

29 

8:32 

8 

6 

32,430 

234 

29 

1  Beginning  of  inversion. 

at  the  12,000-meter  level.  It  may  be  remembered  that  the 
lowest  temperature  experienced  by  Scott  was  about  the 
same. 

A  very  comprehensive  review  of  the  results  of  various 
ascents  made  under  the  auspices  of  the  International  Com- 
mission is  given  by  G.  Nadler  in  the  Beitrage  zur  Physik  der 
freien  Atmosphare,  VI  Band,  Heft  2,  issued  December,  1913. 


48 


THE  PRINCIPLES  OF  A&ROGRAPHY 


20 


FIG.   10.     MONTHLY  VALUES  OF  TEMPERATURES 

The  figures  beneath  the  diagram  (Anz.  d.  and  Aufst.)  indicate  the  number 
of  soundings  upon  which  each  monthly  curve  is  based.  The  figures  at  the 
right  and  left  express  the  altitude  in  kilometers,  and  those  on  the  diagram  itself, 
the  temperatures  in  degrees  C.  For  example,  15  sounding-balloon  ascensions 
in  July  gave  an  average  temperature  of  284°A.  (-j-ll°C.)  at  altitude  one 
kilometer.  The  temperature  decreased  up  to  altitude  12  kilometers  and  then 
remained  almost  constant,  or  increased  slightly  to  altitude  19  kilometers, 
temperature  224°A.  (-49°  C.). 

The  values  doubly  underlined  in  the  table  on  the  following 
page  indicate  the  lowest  temperature  reached,  which  practically 
is  the  beginning  of  the  isothermal  column.  The  commence- 
ment of  this  is  generally  indicated  by  the  symbol  Hc,  and  the 
temperature  at  the  bottom  Te.  The  figures  with  single  under- 
lining show  the  freezing  temperature,  which  descends  to  sea 
level  in  latitudes  62°  north  and  south,  but  which,  at  the 
equator,  is  more  than  5  kilometers  above  sea  level. 

Above  the  equator  the  regular  fall  in  temperature  with 
elevation  continues  to  a  much  greater  height  than  in  temperate 
Variation  of  regions.  Furthermore,  the  lowest  temperatures  in 
stratosphere  the  upper  air  are  found  above  the  equator.  This 
with  latitude  was  indicated  by  de  Bort  and  Rotch  as  early 

as  1905  in  their  exploration  of  the  air  over  the  north  tropical 
Atlantic.  They  also  demonstrated  that  above  the  trade 
winds  there  was  a  change  of  wind  direction ;  that  is,  above  the 


STRATOSPHERE  AND  TROPOSPHERE  49 

northeast  trade  to  southwest  and  above  the  southeast  trade 
to  northwest,  the  height  of  the  plane  of  reversal  varying  greatly, 
while  near  the  equator  the  wind  was  east  at  all  heights.  In 
1908  an  expedition  from  the  Lindenberg  Observatory  sent 
up  balloons  from  Victoria  Nyanza,  on  the  equator,  and  two 
of  these  reached  the  stratosphere,  giving  values  of  17.2  and 
15.4  kilometers  for  Het  and  190°A.  and  203°A.  for  Te.  But 
the  best  confirmation  of  this  variation  with  latitude  is  found 
in  the  records  from  Batavia,  Java,  made  on  December  4,  1913, 
when  a  temperature  of  182°A.  was  recorded.  The  balloon 
reached  a  height  of  26  kilometers,  but  from  17  kilometers 
(the  He)  upward  the  temperature  rose  steadily.  In  six  years, 
1910-1915,  Dr.  van  Bemmelen  obtained  sixty-six  records  out 
of  103  ascents.  The  mean  height  of  the  stratosphere  is  just 


Kilo- 
meters 

Jan. 

Feb. 

Mar. 

Apr 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov 

Dec. 

14 

216°A 

17 

19 

21 

22 

23 

22 

21 

19 

17 

16 

215°A. 

13 

IS 

17 

19 

21 

22 

23 

22 

21 

19 

18 

17 

~16 

12 

17 

18 

19 

20 

21 

22 

22 

21 

19 

18 

17 

11 

17 

17 

17 

19 

20 

21 

21 

20 

19 

18 

10 

20 

20 

20 

22 

24 

25 

26 

26 

24 

23 

21 

9 

24 

23 

24 

26 

29 

31 

34 

33 

33 

31 

28 

25 

8 

30 

29 

30 

32 

36 

38 

41 

41 

41 

38 

35 

32 

7 

37 

36 

37 

39 

42 

45 

47 

48 

47 

45 

41 

38 

6 

43 

43 

44 

46 

49 

52 

55 

55 

54 

51 

49 

45 

5 

50 

49 

50 

52 

56 

59 

61 

62 

61 

58 

55 

52 

4 

57 

56 

57 

59 

62 

65 

67 

68 

67 

64 

61 

58 

3 

63 

62 

63 

65 

68 

71 

73 

74 

73 

70 

67 

64 

2 

67 

66 

67 

70 

73 

76 

78 

79 

78 

75 

72 

69 

1 

71 

71 

73 

76 

79 

82 

83 

83 

81 

79 

75 

72 

Ground 

276 

76 

77 

82 

85 

88 

89 

89 

86 

83 

80 

277 

under  17  kilometers.  At  an  elevation  of  4  kilometers  the  tem- 
perature is  273°A.,  at  10  kilometers  239°A.,  at  17  kilometers 
189°A.,  this  latter  value  based  on  twenty  observations. 

The  following  table  (p.  50)  of  mean  heights  and  temper- 
atures at  the  base  of  the  stratosphere  is  given  by  Gold.1 

The  chart  on  p.  51  (Fig.  11)  drawn  by  Professor  Rotch, 
and  somewhat  modified  from  the  original,  shows  the  height 
of  the  isothermal  stratum  at  various  latitudes  and  corre- 
sponding heights  of  the  isotherm  of  273°A. 

1  Geophysical  Memoirs,  5,  p.  110. 


50 


THE  PRINCIPLES  OF  AEROGRAPHY 


stratosphere 
with  season 


Two  remarkable  features  of  the  stratosphere  are,  then,  the 
great  elevation  in  tropical  regions  and  the  lowered  temperature. 
Variation  of  The  height  of  the  stratosphere  also  varies  with 
the  seasons  and  with  pressure  distribution.  Occa- 
sionally large  variations  are  found  in  Tc  and  H 
from  one  day  to  another.  Thus  Gold  quotes  the  conditions 
on  April  1,  2,  and  3,  1908,  when  the  values  were:  Hc,  10.5  km., 
12.0  km.,  and  7  km.;  Te,  215°A.,  217°A.,  and  224°A. 

MEAN  HEIGHT  AND  TEMPERATURE  OF  THE  BASE 
OF  THE  STRATOSPHERE 


H  (km.) 

T  (°A.) 

Berlin 

10   34 

215 

Munich  
Strassburg 

10.73 

10  72 

216 
215 

Vienna  
Hamburg 

10.33 
10  13 

216 

218 

Paris  
Uccle 

10.55 
10  .  93 

217 
213 

Zurich  

10.2 

218 

Koutchino  
Pavlovsk 

10.5 
9  6 

215 
219 

England 

10.6 

217 

Italy  
Equator  

11.0 
17.0 

214 

182 

Various  explanations  for  the  existence  of  the  stratosphere 
have  been  offered  by  Trabert,  Fenyi,  Gold,  Humphreys, 
Stratosphere  Emden,  Braak,  and  others.  In  general,  the 
due  to  existence  of  the  stratosphere  is  explained  as  due 

radiation  tQ  ra(}iation,  QO^  finding  that  above  the  isobaric 

level  of  250  kilobars  radiation  has  a  heating  effect,  and  below 
this,  a  cooling  effect.  This  hypothesis  is  based  on  the  assump- 
tion that  convective  temperature  equilibrium  exists,  and  on 
the  fact  that  there  is  the  usual  decrease  of  water  vapor  with 
elevation.  The  actual  temperature  of  the  stratosphere,  as 
theoretically  determined  by  Gold,  is: 

203°A.  + 1^(240°  -  203°)  =  212°A. 

The  theory  indicates  that  if  convection  were  absent  and  the 
absorption  of  solar  radiation  did  not  increase  with  height,  the 
normal  state  would  be  one  in  which  the  gradient  of  temperature 
diminished  gradually  to  a  very  small  value.  Emden  derives 


STRATOSPHERE  AND  TROPOSPHERE 


51 


PRESSURE 
KILO  BARS 


MILES 


50 


100 


400 


KILOMETERS 
O 


20 


a  minimum  radiation  temperature  of  214°A.  by  making  allow- 
ance for  a  small  quantity  of  water  vapor.     Braak  holds  that 
the  very  low  temperatures  observed  in  the  upper      Minimum 
part  of  the  tropical  troposphere  (which  are  about      radiation 
30°  lower  than  those  at  the  same  height  in  the      temPerature 
temperate  regions)  must  be  connected  in  the  low-pressure  belts 
of  the  tropics  with  the  rising 
air    currents    of    the    general 
circulation,  which  disturb  the 
temperature    distribution,    as 
determined  by  radiation  and 
absorption,  and  shift  the  trop- 
osphere   to    greater    heights. 
The  fact  that  at  Batavia  the 
upper  limit  of  the  anti-trade 
winds  is  at  the  same  height  as 
the  base  of  the  stratosphere 
proves    that    convection    cur- 
rents   reach    as    high    as    the 
upper  limit  of  the  troposphere. 
Braak    says,    "The   upheaval 
of    the    stratosphere    in    the 
tropics  may  be  demonstrated 
in  a  very  instructive  way  by 
comparison  of  its  height  with 
the   height    of    the   cirrus 
clouds."      The    base    of    the 
cirrus,   in  his  opinion,   repre- 
sents fairly  well  the  height  of 
the  hypothetical  dividing  sur-    Soo 
face  between  the  cooling  and  looc 
heating  effect  of  radiation  for 
moist  air.     The  surface  is  one 
of  fairly  uniform  temperature, 
as    shown    by    the   temperatures 
following   table : 


lS2°A 


30 

28 
20 
24 
22 
20 
18 

10 


FIG.  11.     ROTCH'S  DIAGRAM  OF  HEIGHT 
OF  STRATOSPHERE  WITH  LATITUDE 


at   the   cirrus  level   in   the 


Bossekop,  70°  N.  lat.,  height  of  cirrus  8.3  km.,  temp.  228°A. 
Potsdam,  52°  N.  lat.,  height  of  cirrus  9.2  km.,  temp.  227°A. 
Batavia,  6°  S.  lat.,  height  of  cirrus  11.4  km.,  temp.  225°A. 


52 


THE  PRINCIPLES  OF  A&ROGRAPHY 


At  Blue  Hill,  42°  N.  lat.,  the  average  height  of  cirrus  is  8.9  km. 
Parallel  to  this  surface  or  base  of  the  cirrus  is  the  base  of  the 
stratosphere,  with  a  nearly  constant  temperature  of  2 18° A. 

km. 


J 

I 

* 

\       1 

^        > 

1 

j      : 

J                  J: 

^        S         O        N         1 

3        J 

Y) 

11 

^x""  • 

^ 

|/l\  i 

in 

/s~ 

^«—  + 

X 

' 

^ 

.x^ 

^V/     |    \ 

^ 

0 

9 

After  Gold 

FIG.  12.     ANNUAL  VARIATION  IN  HEIGHT  OF  STRATOSPHERE 


FIG.  13.     HEIGHT  OF  CIRRUS  CLOUDS 


FIG.  14.     HEIGHT  OF  CIRRO-STRATUS  CLOUDS 


kl 

6 
5 
4 

w 

n.               ^                  /               ~\                                \ 

cm 
6 

5 

4 

., 

; 

^"^ 

> 

/ 

i 

^^ 

^Vs 

i 
i 

i 

i 

[ 

_ 

F         FMAMJ         J         ASON         DJ 
FIG.   15.     HEIGHT  OF  CIRRO-CUMULUS  CLOUDS 

Braak  further  points  out  that  there  is  an  essential  difference 
between  the  cloud  formation  in  the  cirrus  level  and  above  it, 

as  compared  with  the  lower  regions,  a  phenomenon 
Behavior  of  F.  .  .  •      i  t, 

cumulus  very  evident  in   the   quiet   tropical   atmosphere. 

cloud  near        ^he    upper    part    of    the    high    cumulus    clouds 

(estimated  at  13  or  14  kilometers)  does  not,  like 

the  lower  part,  dissolve  rapidly,  but,   assuming   a  flattened 

form  and  cirro-stratus-like  appearance,  it  drifts  along  for  a 


STRATOSPHERE  AND  TROPOSPHERE  53 

considerable  time.  This  difference  he  attributes  to  a  cloud- 
dissipating  (cooling)  effect  of  radiation  in  the  lower,  and  a 
cloud-forming  (heating)  effect  in  the  upper,  levels.  He 
believes  that  the  lower  limit  of  the  cirrus  clouds  may  be 
regarded  as  the  level  where,  for  air  of  abundant  water  content, 
the  influence  of  radiation  changes  its  sign. 

Gold "  gives  extensive  comparisons  of  the  various  upper 
clouds  which  may  show  the  same  peculiarities  as  the  annual 
variation  in  the  value  of  He  (Figs.  12-15).  The  actual 
heights,  however,  are  much  less  for  the  clouds,  and  he  is  of 
opinion  that,  while  the  results  point  to  some  common  cause, 
they  indicate  that  the  formation  of  clouds  is  not  a  usual  cause 
of  the  sudden  decrease  and  change  of  sign  in  the  temperature 
gradient. 


CHAPTER  VI 

THE  CIRCULATION  OF  THE  ATMOSPHERE 

18.  Effect  of  the  earth's  rotation  on  the  atmosphere.     If 

the  earth  were  at  rest,  the  general  motions  of  the  atmosphere 
would  be  materially  different  from  what  they  are.  Further- 
more, certain  equations  of  motion  which  are  true  for  small 
eV6^  areas  ^°  not  hold  for  motions  on  a 


Terrestrial 

rotation  larger  scale  covering  a  considerable  area  of  the 

and  air  earth's  surface,  that  is,  the  surface  of  a  rotating 

movements  ,  ,  .         .  , 

sphere.     It  may  be  pointed  out  that,  on  a  sta- 

tionary earth,  warm,  light  air  would  ascend  and  cold,  heavy 
air  descend,  and  thus  the  pressure  would  be  greatest  at  the 
poles  and  least  at  the  equator.  This  is  not  the  case  on 
the  rotating  earth,  where  there  are  great  polar  depressions. 
Moreover,  we  find  that,  in  general,  the  ascending  air  of 
cyclones  is  cold  and  the  descending  air  of  anticyclones 
relatively  warm  and  comparatively  light. 

Our  knowledge  of  the  circulation  of  the  atmosphere  as 
a  whole  is  >still  very  imperfect.  While  the  early  navigators 
The  earliest  doubtless  knew  of  the  existence  of  certain  well- 
knowledge  of  marked  winds,  there  was  no  definite  knowledge 
steady  winds  of  even  guch  steady  wm(}s  as  the  trades  and 

monsoons  until  about  the  end  of  the  seventeenth  century. 

Edmund  Halley,  who  discovered  the  comet  which  bears 
his  name,  was  one  of  the  ablest  physicists  of  his  age  and  was 
a  friend  of  Newton  (it  was  through  Halley  's  efforts  that  the 
Principia  was  published).  Moreover,  he  was  an  explorer 
and  a  navigator.  He  was  the  first1  to  attempt  a  magnetic 
First  definite  survey  (1700)  of  the  ocean,  and  it  is  in  connec- 
knowledge  of  tion  with  his  charts  showing  variation  of  the 

trades  and  compass  that  we  find  advanced  a  theory  of  the 
monsoons  .  .  -  - 

general  and  coasting  trade  winds  and  monsoons 

or  shifting  trade  winds.  And  it  may  not  be  out  of  place,  as 
illustrating  the  constancy  of  the  great  air  streams,  to  quote  a 

1  Gellibrand,  in  1634,  definitely  proved  that  the  compass  direction  varies 
from  year  to  year. 

54 


CIRCULATION  OF  THE  ATMOSPHERE  55 

passage  from  a  lecture1  by  Bauer  showing  that  if  the  magnetic 
survey  vessel,  the  Carnegie,  which  has  circumnavigated  the 
globe  and   repeatedly  intersected  the   course  of     constancy  of 
the    Paramour    Pink,    the    sailing    vessel    which     the  great  air 
Halley  commanded,  were  to  set  course  from  "St. 
Johns,  Newfoundland,  and  follow  the  same  magnetic  courses 
as  those  of  the  Paramour  Pink,  instead  of  coming  to  anchor 
in  Falmouth  Harbor  she  would  have  made  a  landfall  some- 
where on  the  northwest  coast  of  Scotland.     In 
brief,  while  the  sailing  directions  as  governed    by 
the  winds  over  the  Atlantic  Ocean  are  the  same  now 
as  they  were  during  H alley's  time,  the  magnetic  directions  or 
bearings  of  the  compass  that  a  vessel  must  follow  to  reach 
a  given  port  have  greatly  altered." 

In  1735  another  English  astronomer,  Hadley,  advanced  an 
explanation  of  the  trade  winds  based  upon  a  change  of  velocity 
in  air  moving  north  or  south  because  of  change  Hadley's 
in  the  rotational  velocity  of  the  earth  with  dis-  theory  of 
tance  from  the  pole.  Air  moving  from  high  lati-  *  e  tra  es 
tudes  southward  would  pass  to  regions  of  greater  velocity 
and  hence  would  lag  or  apparently  blow  westward.  This 
view  held  for  many  years ;  but  finally  a  flaw  was  found  in  the 
reasoning.  Air  moving  relative  to  the  earth  will,  because 
of  inertia,  maintain  its  absolute  velocity  but  will  change 
its  relative  velocity,  which  is  the  resultant  of  the  absolute 
velocity  and  the  reversed  velocity  of  the  point  of  reference. 

Different  points  on  the  earth's  surface  have  different  abso- 
lute velocities  on  account  of  the  earth's  rotation,  and  hence 
air  would  change  its  relative  velocity  in  moving  The  earth»s 
north  or  south;  but,  as  will  be  shown  later,  the  deflective 
deflecting  force  acts  at  right  angles  to  the  direc- 
tion of  motion.  G.  Coriolis,  in  1835,  gave  the  first  mathe- 
matical solution  of  this  problem  of  the  earth's  deflective 
effect;  but  the  first2  one  to  apply  this  deflection  to  air 
motion  was  William  Ferrel  in  1859,  in  his  general  discussion 
of  the  motion  of  fluids  relative  to  the  earth's  surface. 
Ferrel  showed  that  the  natural  deviation  due  to  the  earth's 

!The  fourth  Halley  lecture,  delivered  at  Oxford,  May  22,  1913;  reprinted  in 
the  Smithsonian  Report,  1913,  entitled  "The  Earth's  Magnetism." 
2  See  also  Charles  Tracy  in  Am.  Jour.  Sci.,  1843. 


56  THE  PRINCIPLES  OF  A&ROGRAPHY 

rotation  could  be  counterbalanced  by  pressure  distribution 
where  the  gradient  was  such  as  to  cause  an  acceleration  suf- 
ficient to  offset  the  effect  of  rotation.  The  deflecting  force 
due"  to  the  earth's  rotation  acts  at  right  angles 
scheme  of  to  the  direction  of  motion.  Ferrel's  scheme  of 

planetary  planetary  circulation  as  later  developed  does 
circulation  1  J  .,  Tj 

not,  however,  meet  modern  views.     It  requires  a 

rather  involved  flow  of  air  from  the  equator  to  the  poles, 
also  in  the  higher  levels;  and  there  is  no  special  evidence 
supporting  such  a  theory  of  this  upper  circulation.  The  cir- 
culation, as  outlined,  requires  marked  depressions  around 
the  poles,  whereas,  in  reality,  an  entirely  different  distribu- 
tion of  pressure  exists. 

Ferrel   gives  an  interesting  illustration  of  the  effect  of  a 
deflecting  force  by  the  experience  of  a  person  walking  over  a 

narrow  drawbridge  while  it  is  turning  on  its  pivot. 
Illustration  ....  .  *  .  , 

of  effect  of  a     If  there  were    no    railings    the    tendency  would 

deflective          foe  to  go  over  the  side;  and  if  there  were  rail- 
ings, to  be  forced  against  them.     This  tendency 
would  be  in  proportion  to    the    velocity    of    transit    across 
the  bridge  and  the  angular  velocity  of  its  gyration.     Some- 
what similar  is  the  deflective  effect  of  the  earth's  rotation. 
A  body  in  motion  in  any  direction  relative  to  the  earth's 
surface   tends,    if  free   to   move,    from  this   direction.     This 
deflecting  force  moves  the  air  to  the  right  in  the  Northern 
Hemisphere    and    to    the    left    in    the    Southern 

Value  of  Hemisphere.     The    deflecting    force    is    usually 

deflective  j     /         ,1         <•  1 

force  expressed    by    the    formula 

2m<j0  V  sin  (f>, 
where 

co  =  angular  velocity  of  earth's  rotation 

2* 


86,164  sec. 


=  0.00007292; 


V,   the  velocity  with  which  the  body  is  moving  relative  to 
the  earth;  0,  the  latitude;  and  m,  the  mass. 

Briefly  stated,  moving  air  is  deflected  to  the  right  in  the 
Northern  Hemisphere  and  to  the  left  in  the  Southern  Hemi- 
sphere. Components  of  gravity  cause  the  deflections.  For 


CIRCULATION  OF  THE  ATMOSPHERE  57 

east  and  west   motions   the  direction  but  not   the  velocity 
is  changed;  while  for  north  and  south  motion,  the  velocity  is 

affected.     Air  moving  toward   the  pole  has  its 

..          i      •,         •  j  j        •  •  Deflection 

eastward    velocity    increased,    and    air    moving        Of  moving 

toward  the  equator  has  its  velocity  diminished. 
It  will  be  well  to  recall  the  law  of  equal  areas 
or  constancy  of  angular  momentum;  also  the  law  of  con- 
stancy of  density  multiplied  by  velocity,  sometimes  called 
Egnell's  law,  or  Clayton's  law. 

If  there  is  no  velocity, —  that  is,  if  it  is  calm  and  there  is 
no  air  movement, —  there  is  no  acceleration.  Again,  at  the 

equator  the  sin  </>  is  zero.     The  factor  co,  or  the 

•    ,,  ,  i      •,        •    £         j  i        -r    -j.        ,i  Application 

earth  s  angular  velocity,  is  found  by  dividing  the        Of  formula 

circumference  by  the  number  of  seconds  in  one  !or  deflect- 
sidereal  day.  It  may  also  be  pointed  out  here 
that  acceleration  due  to  centripetal  force  must  be  deducted 
from  gravity,  and  the  remaining  component  of  gravity, 
called  apparent  gravity,  or  gravity  as  ordinarily  measured, 
is  somewhat  less  than  absolute  gravity,  as  calculated  by 
astronomers;  and  it  acts  in  a  slightly  different  direction.1 

Ekholm  sums  up2  the  chief  deviations  due  to  this  deflective 
effect  as  follows : 

"In  the  Northern  Hemisphere  in  an  obliquely  upward  cur- 
rent the   horizontal  component  of   the  deviating  force  of  a 
south  wind  is  less,  and  that  of  a  north  wind  is     Ekholm's 
generally  more,  than    in    a    horizontal    current,      summary 
In   an   obliquely   downward  current  the  reverse      due^* 10 
holds  good.     As  to  the  Southern  Hemisphere,  we     deflective 
need  only  to  interchange  the  north  and  south  in     effect 
the  above  proposition.    .    .     .     The  north  and  south  winds 
are  never  vertically  deviated. 

"In  an  east- west  current,  if  the  air  is  obliquely  rising  or 
falling,  the  deflective  force  will  not  be  directed  exactly  per- 
pendicular to  the  velocity ;  but  if  the  air  is  rising  it  throws  the 
east  winds  a  little  forward  and  the  west  winds  a  little  back- 
ward; and  the  reverse  of  this,  if  the  air  is  falling.  The  west 

1 A  geometric  method  of  deriving  the  deflective  force  due  to  the  earth's 
rotation  is  given  by  T.  Okada  in  Monthly  Weather  Review,  May,  1908,  p.  147; 
also  by  C.  F.  Marvin,  ibid.,  October,  1915,  p.  503. 

2  Monthly  Weather  Review,  June,  1914,  p.  330. 


58  THE  PRINCIPLES  OF  ARROGRAPHY 

winds  always  deviate  upward  and  the  east  winds  downward." 
The   following    example    may    illustrate    the    deviation   in 
absolute  units : 

''If  the  air  temperature  is  290°A.  and  the  pressure  1,000 
kilobars,  and  the  density  of  the  air  equal  to  0.0012,  the 
acceleration  will  be  equal  to  the  gradient  g;  that  is,  the 
acceleracion  produced  by  g  will  be  expressed  by  the  same 

measure  of  length  as  the  gradient  itself,  if  this  is 
Relation  .  •  -,•  1  TVT  1  •> 

between  given  for  a  meridian  degree.     Now  we  know  by 

gradient  and  observation  the  relation  between  gradient  and 
wind  velocity  .  1  .  1  , .  T  t 

wind  velocity  in  a  steady  motion.     In  a  cyclone, 

for  example,  a  gradient  of  5  millimeters  will  generally  produce 
stormy  winds  with  a  velocity  of  about  30  meters  per  second 
(v=  3,000  cm. /sec.).  Then,  since  o>  =0.00007292,  we  get  for 
a  current  parallel  to  the  equator  a  deflecting  force  equal  to 
0.44  cm. /sec.2,  which  is  nearly  the  same  value  as  that  for  the 
acceleration  produced  by  the  gradient,  this  latter  being  0.5 
cm. /sec.2"  The  discussion  is  carried  further  by  Ekholm,  not 
omitting  the  effect  of  friction  (the  coefficient  of  friction,  k, 

ranges  from  0.00002  to  0.00004  for  the  open  sea, 
Effect  of  anc[  does  not  exceed  0.00012  for  a  very  rough  con- 

on  deviation  tinental  surface ;  and  the  friction  would  therefore 

be  kv,  or  less  than  the  deflecting  force  in  a  current 
parallel  to  the  equator).  "Friction  in  a  horizontal  parallel  cur- 
rent is  so  small  in  the  upper  strata  that  it  is  negligible  compared 
with  the  gradient.  Hence  it  follows  that  in  the  upper  strata 
even  a  very  slight  gradient, — for  example,  0.01  centimeter,— 
if  it  act  sufficiently  long  in  the  direction  of  motion,  can 

produce  a  marked  velocity,  particularly  as  the 
Acceleration  .  .  .  I-  1  ±  ^  j  •.*. 

produced          acceleration  is  inversely  proportional  to  the  density 

bj  diSHght  of  the  air'  Thus  at  an  altitude  where  the  density 
is  only  half  that  at  sea  level  (about  8  kilometers, 
or  the  average  height  of  cirrus)  the  acceleration  produced  by 
the  above-named  slight  gradient  will  be  0.03  cm. /sec.2  This 
acceleration  acting  in  the  direction  of  motion  during  100,000 
seconds  or  28  hours  will  cause  a  velocity  of  3,000  cm. /sec.  or 
30  meters  per  second;  and  with  this  velocity  the  deflective 
force  would  be  0.44  cm. /sec.2,  or  about  15  times  greater  than 
the  component  of  acceleration  acting  in  the  direction  of  motion. 


CIRCULATION  OF  THE  ATMOSPHERE  59 

"The  horizontal  component  of  the  deviating  force  attains 
its  greatest  value  in  the  higher  latitudes;  and  the  vertical 
component  in  lower  latitudes." 

19.  Impressions  of  gyroscopic  motion.     Sandstrom  makes 
the  pertinent  suggestion1  that  much  of  the  difficulty  experi- 
enced in  comprehending  gyroscopic  motion  is  due  primarily 
to  the  absence  of  any  specific  sense  that  would 
enable  us   to  detect,  if  not  to  feel,   the  earth's      motion°P 

rotational  effects.     He  says:  "From  childhood  on,      difficult  to 

.,    ,  -.    ,.,          .-11  •  comprehend 

we  are  accustomed  to  regard  the  visible  portion 

of  the  earth's  surface  as  at  rest.  To  be  sure,  we  all  really 
know  that  the  earth  does  rotate  and  we  can  imagine  this, 
but  we  have  no  sense  by  which  to  feel  it.  .  .  .  The 
constant  deception  that  the  earth  is  at  rest  impresses  the 
observer  as  being  the  true  state  of  affairs.  Under  these  con- 
ditions it  is  indeed  very  natural  that  even  the  effects  of  the 
earth's  rotation  should  appear  foreign  to  us."  And  again: 
"A  being  who  could  feel  the  terrestrial  rotation  would  prob- 
ably find  it  very  natural  that  specifically  heavy  air  should 
ascend,  and  light  air  descend.  That  person  would  find  it  easy 
to  apply  Coriolis'  theorem,  because  the  resulting  conclusions 
would  be  in  harmony  with  his  sensations." 

The  best  means  of  bringing  home  the  effects  of  rotation 
upon  the  air  and  water  of  the  earth  are  hydrodynamic  experi- 
ments   with    rotating    vessels.     Sandstrom    con-      Hydro- 
siders  these  experiments  from  two  hypothetical      dynamical 
standpoints.     The  first  would  be  that  in  which      w1thramentS 
a    very    small    individual    was    on    the    rotating      rotating 
vessel,   a  being  so  small  and  with  such  limited      vesse 
observational  powers  that  he  could  not  recognize  the  rotation 
of   the   vessel.     To   such   a   person    many   of    the    processes 
would    appear  inexplicable.      For  example,    by    experiments 
carried  out  on  a  small  scale  he  would  find  that 
liquids  and  gases  specifically  lighter   than   their     fonSdered*8 
surroundings  will  strive  upward,  but  would  find     from  the 
just  the  opposite  in  experiments  on  a  large  scale.     Jointof  view 
Eventually,   by  numerous  experiments  he  might 
come  to  the  conclusion  that  the  vessel  had  a  rotating  motion. 

1  Monthly  Weather  Review  Sept     1914,  p.  523. 


60  THE  PRINCIPLES  OF  A&ROGRAPHY 

With  mathematical  analysis  he  might  work  out  a  theorem, 
such  as  Ferrel's  or  Coriolis'. 

The  second  viewpoint  would  be  that  in  which  the  observer 
saw  the  rotating  vessel  and  the  rotary  movements.  This 
second  method  would  present  matters  as  they  would  appear 
to  an  observer  outside  of  the  earth.  Studying  oceanic  and 

atmospheric  circulations,  he  would  see  a  series  of 
Experiments      ,  .      1  .  1 

from  the  large  vorticular  motions  and  could  comprehend 

absolute  _  ^e  effects  of  centrifugal  forces.  Sandstrom  em- 
pomt  of  view  .  .  1  .  n  .  1^11 

phasizes  the  advantage  of  this  second  method, 

since  "one  may  thereby  see  and  judge  of  the  whole  absolute 
motion  and  the  associated  forces  without  any  intermediary. 
In  the  former  method  one  is  concerned  with  two  motions: 
the  relative  motions  of  the  atmosphere  or  of  the  ocean 
water  as  referred  to  the  earth's  surface,  and  the  motions  of 
the  earth's  surface  as  the  result  of  its  rotation.  The  com- 
pounding of  these  two  motions  and  of  the  forces  that  bring 
them  into  existence  is  not  always  an  easy  problem." 

He  suggests  that  the  experiments,  as  outlined  below, 
be  considered  first  from  the  absolute  (that  is,  the  second) 
point  of  view  and  then  be  transferred  to  the  relative  (first) 
point  of  view.  In  this  way  he  thinks  that  we  may  gradually 
acquire  the  ability  intuitively  to  take  immediate  account  of 
the  effect  of  terrestrial  rotation  when  discussing  the  observed 
movements;  and  he  considers  this  acquirement  as  one  of  the 
most  important  objects  of  dynamic  meteorology. 

To  explain  the  heaping  up  of  warm  ocean  water  in  the 
horse  latitudes,1  he  fills  a  glass  vessel  30x10x10  centimeters 
to  a  depth  of  about  3  centimeters  with  fresh  water,  and  intro- 
duces   gently   below   the   fresh   water   an    equal 
The  heaping  &     ,    J 

of  ocean  volume    of    salt  water.     One    of    the    strata    is 

explained          colored  for  purposes  of  better  observation.     By 

means  of  a  bellows,  a  tube,  and  the  perforated 

spout  of  a  watering  pot,  a  current  of  air  is  forced  downward 

upon  the  water  surface  as  in  Fig.  16.     At  once  it  becomes 

1  This  name  has  its  origin  in  the  fact  that  in  early  days  vessels  bound  from 
New  England  to  the  West  Indies,  carrying  horses,  were  so  often  delayed  by 
calms  that  for  want  of  water  it  was  necessary  to  throw  the  horses  overboard. 
These  calms  are  experienced  between  the  prevailing  westerly  winds  north  of 
25°  N.  latitude  and  the  northeast  trade  winds. 


CIRCULATION  OF  THE  ATMOSPHERE 


61 


FIG.  16. 


EFFECT  OF  RADIALLY  DIRECTED  WINDS 
UPON  A  SYSTEM  AT  REST 


clear  that  the  bounding  surface  between  the  two  strata  bulges 
upward,  a  result  of  the  downward  air  current  driving  the 
surface  water  toward  the  sides  of  the  vessel.  If  the  vessel  is 
now  placed  on  a  rotating  table,  and  a  slow  rotation  around 
a  vertical  axis  begun, 
it  will  be  found  on 
using  the  bellows  as 
before  that  the  bound- 
ing surface  of  the  two 
layers  does  not  bulge 
upward  at  a  point 
beneath  the  spout, 
but,  on  the  contrary, 
is  depressed,  as  shown 
in  Fig.  17.  From  the 
experiment  one  may 
conclude  that  the 
heaping  of  ocean  water 
in  the  horse  latitudes 
is  a  result  of  the  earth's 
rotation  in  combina- 
tion with  the  anti- 
cyclonic  condition's 
prevailing  there. 

Sandstrom  then  endeavors  to  explain  the  phenomenon 
from  the  viewpoint  of  the  small  imaginary  being  referred  to 
above,  who  can  observe  only  the  air  movement  relative  to 
the  rotating  vessel  and  perceives  only  that  the  air  blows 
spirally  outward  from  a  center  (Fig.  18).  This  air  move- 
ment produces  also  an  anticyclonic  circulation  in 
the  water,  whereby,  on  account  of  the  deflective 
force,  this  rotation  comes  into  action  and  drives 
the  water  toward  the  right  hand.  In  other  words,  it  presses 
toward  the  center,  and  the  water  heaps  up  at  the  center. 
From  the  other  viewpoint  it  is  seen  at  once  that  the  vessel 
is  rotating  and  that  a  stream  of  air  is  blowing  radially  from 
a  central  point  upon  the  water  surface  (Fig.  19).  The 
lower  stratum  has  the  same  velocity  of  rotation  as  the  vessel 
itself,  but  the  rotation  of  the  surface  water  is  hindered  by  the 


FIG.  17. 


EFFECT  OF  RADIALLY  DIRECTED  WINDS 
UPON  A  ROTATING  SYSTEM 


Anticyclonic 
circulation 


62 


THE  PRINCIPLES  OF  AEROGRAPHY 


\ 


FIG.    18.     RELATIVE    MOTION   OF   RADIALLY 

DIRECTED  WINDS  AT  THE  SURFACE  OF 

A  ROTATING  SYSTEM  (VIEWED 

FROM  ABOVE) 


\ 


\ 


radial  currents,  and  the  surface  water  moves  more  slowly  than 
the  vessel.     The   centrifugal   force   of   the   lower  stratum  is 

r     greater    than    that   of 

the  upper.  Hence  the 
lower  water  is  forced 
outward  while  the  up- 
per stratum  collects  in 
the  center. 

In  consequence  of 
the  earth's  turning  on 
its  axis  its  surface  is 
everywhere  in  cyclonic 
rotation  and  the 
velocity  of  rotation  is 
determinable  by  the 
Foucault  pendulum 
experiment.  If  the 
earth's  angular  veloc- 
ity of  rotation  is  co,  then 
the  angular  velocity  of 

\any  point  on  the  earth '  s 
surface  is  co  sin  <£,  where 
</>    is    the    geographic 
latitude.     The  time 
required  by  the  earth 
to    complete    one   rotation    is    24   hours.     Air,   then,   that    is 
apparently  at  rest,  has  a  cyclonic  circulation;  in  fact,  even  the 
air  in  the  familiar  anticyclonic  whirl  actually  pos- 
sesses a  cyclonic  rotation.     Air,  then,  apparently 
at  rest,  is  subject  to  a  centrifugal  force  which  is 
reinforced    under  cyclonic    conditions   and  weakened   under 
anticyclonic  circulation.    These  facts  explain  the  temperature 
distribution  within  cyclones  and  anticyclones.     In  a  cyclone 


/    \     N 


FIG.    19.     ABSOLUTE    MOTION   OF    RADIALLY 

DIRECTED  WINDS  AT  THE  SURFACE  OF 

A  ROTATING  SYSTEM  (VIEWED 

FROM  ABOVE) 


Cyclonic 
movement 


Velocity 


the    rotation    at    first    increases,     with    altitude 


increases  with  reaching  a  maximum  at  a  certain  height.  The 
elevation  centrifugal  force  is  greatest  at  that  level,  and 

the  air  is  driven  most  strongly  outward.  The  air  therefore 
undergoes  dynamic  cooling  below  the  level  of  maximum 
cyclonic  rotation  and  dynamic  warming  above  that  level. 


CIRCULATION  OF  THE  ATMOSPHERE  63 

In  an  anticyclone  the  anticyclonic  rotation  has  its  maximum 
at  a  certain  level  where  the  centrifugal  force  is  lowest,  and 
from  that  level  the  centrifugal  force  increases  both  upward 
and  downward.  Consequently,  below  this  level  the  air  will 
be  drawn  downward,  and  above,  it  will  be  drawn  upward; 
therefore,  below  this  level  the  air  will  be  warmed  dynamically, 
and  above,  it  will  be  cooled. 

Again,  the  west-east  drift  of  the  atmosphere  in  middle  and 
higher  latitudes  forms  a  gigantic  polar  cyclone.  This  west- 
east  drift  has  its  maximum  at  a  certain  level  and  West-east 
diminishes  both  upward  and  downward  there-  drift  of  the 
from.  At  the  level  of  the  maximum  drift  the  atmosPhere 
centrifugal  force  is  the  greatest;  below  that  level  the  air  of 
the  polar  regions  is  drawn  upward,  and  above  it  the  air  is 
drawn  downward;  therefore,  beneath  this  level  the  tempera- 
ture of  the  air  at  the  poles  is  lower  than  it  is  at  the  equator, 
while  above  this  level  the  air  is  warmer  above  the  region  of 
the  poles  than  it  is  at  the  same  level  over  the  equator. 

In  1853  Coffin,  studying  the  winds  of  the  globe,  noted  that 
on  the  left  of  prevailing  winds  the  pressure  was  low;  and  in 
1857  Buys-Ballot  discovered  the  law  that  if  you 
stand  "with  your  back  to  the  wind,  the  pressure 
is  low  on  the  left  and  high  on  the  right."  In 
1860  he  pointed  out  the  connection  between  the  above  relation 
and  the  rotation  of  the  earth;  and  later  (1832),  when  Buchan 
mapped  the  distribution  of  pressure  over  the  surface  of  the 
globe,  the  close  relation  between  wind  and  barometric  gradient 
was  clearly  shown.  Guldberg  and  Mohn  (1876),  Sprung, 
Weihrauch,  Ekholm,  W.  M.  Davis,  and  others  have  written 
on  the  subject.  Shaw,  however,  was  the  first  (1893)  to  point 
out  the  necessity  for  calculating  and  utilizing  gradient  veloc- 
ities. Largely  because  of  the  experiments  of  Dines,  it  has 
become  evident  that  the  change  in  the  velocity,  with  height, 
brings  these  upper  currents  more  and  more  into  Balance  of 
agreement  with  the  theoretical  wind  computed  barometric 
from  the  surface  gradient.  Gold,  in  1908,  showed 
that  the  wide  separation  of  isobars  in  the  inner 
regions  of  an  anticyclone,  as  compared  with  the  closeness  of 
the  lines  in  the  central  region  of  a  cyclone,  was  dependent  upon 


64  THE  PRINCIPLES  OF  A&ROGRAPHY 

the  fact  that  in  an  anticyclone  the  curvature  gradient  acts  in 
the  opposite  sense  to  the  general  rotation  gradient,  whereas 
in  a  cyclone  the  curvature  gradient  and  the  rotation  gradient 
are  concurrent.  Thus,  says  Shaw,  "the  idea  of  the  balance 
of  barometric  gradient  by  velocity,  as  a  primary  law  of  atmos- 
pheric circulation,  has  been  gradually  strengthened." 

NOTE. — A  schematic  representation  of  the  forces  causing  and  modifying 
winds  may  be  found  in  the  Introductory  Note,  by  Professor  C.  F.  Marvin,  to 
Weather  Forecasting  in  the  United  States,  W.  B.  583,  issued  in  1916.  On  pp.  22 
and  23  numerical  data  are  given. 


CHAPTER  VII 

THE  MAJOR  CIRCULATIONS 

20.  Hyperbars  and  infrabars.     There   are   at  least  three 
causes  operating  in  the  establishment  of  the  major  circula- 
tions of  the  globe  and  less  directly  in  the  minor       Causes  of 
circulations.     These  are  (1)  the  unequal  heating       the  major 
of  equatorial  and  polar  regions  (and,  in  the  last 
analysis,  gravity  is  the  prime  factor  in  causing  air  motion) ; 
(2)  the  deflective  forces  due  to  the  earth's  rotation;  and  (3) 
the  unequal  absorption  of  heat  by  land  and  water  surfaces, 
which,  as  we  shall  see  later,  determines  largely  the  location 
of  the  hyperbars  and  infrabars,   or  centers  of  action.     The 
distribution  of  pressure  over  the  continents  and  oceans  deter- 
mines in  large  measure  the  path  and  frequency  of  smaller 
circulations.     Teisserenc  de  Bort,   in   188 1,1  gave  the  name 
"grand  centers  of  action"   to  certain  areas  of  high  and  low 
pressure,  and  traced  a  relation  between  the  posi-         "Grand 
tion  of  these  centers  and  periods   of   abnormal         centers  of 
temperature.     De    Bort    thus    explained    certain 
abnormal  winters;  a  relation  which  had  been  suggested  by 
Hoffmeyer,  in  1878,  and  which  has  since  been  discussed,  by 
Fassig  in  connection  with  abnormal  Marches  on  the  Atlantic 
coast,    by  McAdie  in   connection   with   winter   rain    on    the 
Pacific  coast,  and  by  Humphreys  on  warm  and  cold  winters 
in  the  eastern  part  of  the  United  States. 

Beginning  with  the  largest  ocean,  the  Pacific,  we  find  two 
well-marked  hyperbars,  or  areas  where  the  pressure  is  in  excess. 
One  is  west  of  California  and  extends  southwest ;  Areas  where 
and  the  other  is  over  the  southern  Pacific,  west  of  the  pressure 
Chile.  There  are  also  certain  continental  hyper- 
bars to  which  reference  is  made  below.  Over  the  Atlantic 
there  are  two  hyperbars,  a  small  Bermuda  one  and  a  larger 
one,  west  of  southern  Africa,  extending  from  15°  E.  to  30°  W. 

1  "Etude  sur  Thiver  de  1879-1880,"  Ann.  du.  Bur.  Cent.  Meteorol.  de  France, 
IV,  1881. 

6  65 


66  THE  PRINCIPLES  OF  A&ROGRAPHY 

and  from  10°  S.  to  40°  S.  The  fifth  hyperbar  is  over  the 
Indian  Ocean.  The  land  hyperbar  s  are  over  western  North 
America,  southwestern  Europe,  and  central  Asia.  There 
are  certain  peculiar  reversals  in  these  areas  between  summer 
and  winter,  as,  for  example,  the  North  American  hyperbar, 
which  becomes  an  infrabar  in  summer;  and  the  Australian 
hyperbar  of  July  (the  winter  season),  which 
becomes  an  infrabar  in  January  (the  summer 
season  for  those  latitudes).  The  more  prominent 
infrabars,  or  areas  of  diminished  pressure  at  the  surface,  are 
the  Aleutian  of  the  North  Pacific  and  the  Icelandic  of  the 
North  Atlantic.  Thus,  in  a  general  way,  we  may,  following 
Buchan,  place  the  winter  hyperbars  in  the  northern  hemisphere 
Position  of  between  20°  and  40°,  except  over  land  surfaces 
winter  where  they  extend  farther  north.  In  the  southern 

yper  ars  hemisphere  the  hyperbars  are  more  evenly  aligned, 
and  we  find  them  like  peaks  in  the  belt  of  prevailing  high 
pressure  between  latitudes  20°  and  40°. 

Humphreys  has  advanced  the  view  that  hyperbars  occur 
only  where  cold  ocean  currents  cross  the  belts  of  high  pressure. 
Relati  n  ^^S  explanati°n>  however,  overlooks  the  con- 

between  tinental    hyperbars.     The    general    character    of 


persistency       ^e    season  —  such    as    the    predominant    wind, 

of  hyperbars  •    r  n         i  u 

and  char-         temperature,    or   rainfall  —  has    been    connected 

acter  of  by  various  writers  with   the  persistency  of  the 

hyperbars.     There  seems  to  be  some  correlation 

between  abnormal  weather  and  the  shifting  of  these  pressure 

areas.     Hann  showed  that  the  pressure  changes  between  the 

Azores  hyperbar  and  the  Icelandic  infrabar  were  of  unlike 

character,  rising  pressure  in  one  being  attended  with  falling 

pressure  in  the  other;  and  falling  pressure  in  the  Iceland  area 

causing  warmer  weather  over  central  and  northwestern  Europe. 

In  various  papers1  published  in  recent  years  attention  has 

iFassig,  Am.  Jour,  of  Sci.,  Nov.,  1899;  McAdie,  "Climatology  of  California," 
U.  S.  Weather  Bureau,  Bulletin  L,  1903;  Lockyer,  Science  Progress,  Oct.,  1906, 
No.  2;  Okada,  Central  Meteorological  Observatory  Bulletin,  No.  4,  Tokyo,  Japan, 
1910;  Humphreys,  "Origin  of  the  Permanent  Ocean  Highs,"  Bulletin  U.S.  Weather 
Bureau,  Oct.,  1911;  Monthly  Weather  Review,  Dec.,  1914;  McAdie,  "Forecasting 
the  Water  Supply  in  California,"  Monthly  Weather  Review,  July,  1913,  also 
Bulletin  Mount  Weather  Observatory,  Dec.,  1910,  and  Monthly  Weather  Review, 
April,  1908. 


THE  MAJOR  CIRCULATIONS  67 

been  directed  to  the  relation  between  pressure  distribution 
and  the  character  of  the  season.     On  the  Pacific  slope  typical 
wet  winters  occur  when  the  North  Pacific  infrabar          ,   . 
overlies  the  continent  west  of  a  line  drawn  from       between 

Calgary  to  San  Francisco.     Typical  dry  winters       pressure 

.,,.,.  j  f     ,  distribution 

are  associated  with  a  westward  extension  of  the       and  char- 
continental   hyperbar    to    the    coast    line  and  a       acter  of 
retreat  of  the  Aleutian  infrabar  to  the  northwest. 

In  a  normal  season  the  Aleutian  infrabar  extends  from 
latitude  40°  N.  to  60°  N.  and  from  longitude  130° W.  to 
140°  E. 

In  summer  months  the  distribution  of  pressure  changes. 
The  Aleutian  infrabar  practically  disappears.  The  continental 
hyperbar  is  displaced  somewhat  eastward,  and  the  oceanic 
hyperbar  moves  farther  north.  Summer  in  California  is 
practically  rainless,  and  there  are  strong  west  and  northwest 
winds. 

The  accompanying  charts  (Figs.  20  and  21,  p.  68)  show 
the  pressure  distribution  during  selected  dry  and  wet  weather 
months.  Fig.  20  shows  the  sea-level  pressure  and  surface 
winds  during  January,  1902,  typical  of  a  dry  winter  month. 
Other  dry  winter  months  were  January,  1898  and  1904, 
February,  1899,  1890,  and  1912,  and  March,  1898,  1901, 
and  1908. 

Fig.  21  shows  the  sea-level  pressure  and  surface  winds 
during  February,  1902,  which  are  typical  of  a  wet  winter 
month.  Other  wet  winter  months  were  January,  1906, 
1907,  1909,  1911,  February,  1904  and  1909,  and  March, 
1904,  1907,  and  1911. 

The  precipitation  during  January,  1902,  a  dry  winter  month, 
was  so  scant  that  the  deficiency  in  water  supply  was  approxi- 
mately 40,000  million  cubic  meters.  The  precipitation  during 
February,  1902,  was  so  heavy  that  the  excess  of  water  supply 
for  the  month  amounted  to  approximately  53,000  million 
cubic  meters. 

It  is  also  to  be  pointed  out  that  the  frequency  and  path  of 
individual  disturbances  depend  primarily  upon  the  strength 
and  location  of  the  larger  areas.  Individual  lows  move 
rapidly  southward  when  the  continental  high  overlies  Idaho, 


68 


THE  PRINCIPLES  OF  AEROGRAPHY 


FIG.  20.     TYPICAL  PRESSURE  DISTRIBUTION  AND  WIND  DURING  A 

DRY  WINTER  MONTH 
The  hyperbar,  1024  kilobars  (30.25  inches),  favoring  north  winds  over  California 


FIG.  21.     TYPICAL  PRESSURE  DISTRIBUTION  AND  WIND  DURING  A 

WET  WINTER  MONTH 
The  infrabar,  1007  kilobars  (29.75  inches),  favoring  south  winds  over  California 


THE  MAJOR  CIRCULATIONS  69 

eastern  Washington,  and  eastern  Oregon.1  Similarly,  when 
the  Aleutian  infrabar  extends  well  southward,  individual  lows 

deepen  rapidly  and  move  south  rapidly.     Under    . 

•u  j-1-  <L-U          •  -11        \.      j    £  Relation  of 

such  conditions  the  ram  area  will   extend  from    individual 

the  Washington  coast  to  the  northern  California    disturbances 
-,         t  ,  -  ,  .to  large  areas 

coast  in  twelve  hours,   to  the  central  coast  in 

twenty-four  hours,  and  to  the  coast  south  of  Point  Concep- 
tion in  thirty-six  hours. 

For  the  Atlantic  coast,  Humphreys  has  advanced  the  view 
that  unusually  mild  winters  are  determined  by  the  persistence 

of  the  Bermuda  hyperbar,  while  continued  absence 

£  ...  n.     •  ^         11      1  Effect  of 

ot  this  results  in  exceptionally  low  temperatures.     Bermuda 

Low  surface  temperature  in  the  vicinity  of  the    hyperbar  on 
r>  j  j  j  ,1  j     Atlantic  coast 

Bermudas  may  depend  upon  the  temperature  and 

strength  of  the  Labrador  current,  though  it  is  more  likely,  as 
we  shall  see  later,  that  ocean  currents  are  the  result  rather 
than  the  cause  of  pressure  displacement.  Humphreys  holds 
that  "a  persistent  strong  Labrador  current  would  seem  to 
indicate  a  subsequent  (after  a  fortnight  or  longer  period) 
development  of  a  more  or  less  equally  persistent  Effect  of 

Bermuda   high,    and   through   it   the   prevalence         Labrador 
...  •    ,         V      1      •      1  current  on 

during  winter  of  relatively  warm  weather  through-         Bermuda 

out  the  eastern  United  States.  On  the  other  hyperbars 
hand,  a  long-continued,  weak  Labrador  current  would  indi- 
cate the  subsequent  absence  of  Bermuda  highs  and  the 
prevalence  over  the  eastern  United  States  of  unusually  low 
temperatures." 

Ward2  has  discussed  at  some  length  the  cyclonic  and  anti- 
cyclonic  control  of  the  weather  of  the  United  States.     Thus 
spring     and    autumn,     transition     periods,     are 
marked  by  the  struggle  between  cyclonic  and  solar 
controls,    and    hence    by    striking    convectional     phenomena 
phenomena.     As  summer  passes,   the  sun's  rays     autumngan 
become  more  and  more  oblique  and  the  control 
of  the  weather  passes  gradually,  but  irregularly,  from  the 
sun  back  again  to  the  cyclone. 

1For  weather  maps  illustrating  movements  of  California  Lows,  see  E.  A. 
Beals  in  Weather  Forecasting  in  the  United  States,  pp.  327-335;  also  G.  H.  Will- 
son,  ibid.,  pp.  335-339. 

2  Annals  of  the  Assoc.  of  Am.  Geog.,  Vol.  IV,  pp.  3-54. 


70  THE  PRINCIPLES  OF  A&ROGRAPHY 

21.  The  effect  of  ocean  currents  on  atmospheric  circulation. 

Ocean  currents  belong  more  properly  in  a  treatise  on  hydrog- 
raphy, and  are  appropriately  discussed  in  the  publications  of 
nautical  institutes,  coast  surveys,  and  hydrographic  offices  con- 
nected with  the  navy  departments  of  various  nations.  There  is, 
however,  a  close  connection  between  some  of  the  great  currents 
of  the  ocean  and  the  great  air  streams.  In  water,  as  in  air, 
there  are  convectional  currents  and  displacements  due  to  differ- 
ences in  density  caused  primarily  by  temperature 
currents  like  inequalities ;  but  in  water  there  are  also  density 
air  currents  differences  caused  by  salinity  and  the  origin  of 
respects  the  water.  Great  ocean  currents,  like  great  air 

currents,  are  deflected  by  the  earth's  rotation; 
and,  furthermore,  the  great  sea  currents  are  very  often 
(certainly  so  far  as  surface  conditions  go)  driven  by  the 
great  wind  systems.  In  Figs.  22  and  23,  which  are  essen- 
tially Koppen's  wind  charts,  one  may  get  a  fair  idea  of  the 
surface  movement  of  the  ocean. 

Unlike  air  currents,  ocean  currents  may  be  divided  into 
two  main  classes,  warm  and  cold,  and  of  these  the  latter 
are  probably  the  more  active  in  originating  and  maintaining 
T  .  ocean  circulation.  Therefore  it  is  proper  to 

classes  begin  with  the  cold  Antarctic  and  Arctic  currents. 

currents  ^  ^  south  pole  there  is,  as  the  recent  exploring 

expeditions  have  shown,  a  large  continent  or  land 
mass  having  an  average  elevation  of  2,200  meters  above  sea 
level.  Even  in  the  summer  months  (December,  January, 
and  February)  the  average  temperature  is  below  273°A.  In 
fact,  this  isotherm  almost  coincides  with  the  Antarctic  Circle 
(lat.  66°  33'  S.),  and  this  notwithstanding  that  the  earth  is 
then  in  perihelion.  In  spite  of  greater  insolation,  the  farther 
poleward  one  goes  the  lower  the  temperature  falls.  The 
Temperature  lowest  mean  annual  temperature,  that  of  Fram- 
of  Antarctica  heim  headquarters  (at  sea  level,  78°  38'  S.,  164° 
30'  W.),  is  248°A.  (-13° P.),  the  lowest  mean  on 
record.  Meinardus  has  called  attention  to  the  fact  that  the 
east  winds  south  of  the  trough  of  low  pressure,  which  were 
formerly  supposed  to  be  of  Antarctic  origin  and  have  the 
character  of  boreal  winds,  are,  on  the  contrary,  moist,  warm., 


THE  MAJOR  CIRCULATIONS 


71 


snow-producing 
winds.  Except 
for  the  humidity 
and  marked  pre- 
cipitation  it 
might  naturally 
be  thought  that 
these  were  foehn 
winds.  In  this 
region,  of  course, 
the  circulation  of 
the  winds  in  a 
cyclone  is  clock- 
wise, or  in  an  op- 
posite direction 
from  that  in  the 
northern  hemi- 
sphere. 

The  chief  fao 
tor  affecting 
oceanic  circula- 
tion is  the  con- 
stant flow  of  ice 
from  the  interior 
of  Antarctica. 
There  would  ap- 
pear to  be  a  con- 
stant excess  of 
precipitation  over 
evaporation  and 
consequently  a 
continuous  run- 
off of  this  snow 
in  the  form  of  gla- 
ciers and  marginal 
ice  fields.  In  the 
summer  some  of 
this  ice  is  carried 
away  by  the 


72 


THE  PRINCIPLES  OF  AEROGRAPHY 


southern  ocean 
current,  which 
is  perhaps  the 
greatest  of  all  sea 
currents,  flowing 
east  around  the 
world  in  latitude 
60°  S.  Flowing 
poleward  and 
crossing  the  main 
current  are  the 
Madaga  scar, 
Australian- 
counter,  and 
Argentine- 
counter  currents, 
while  flowing  to- 
ward the  equator 
are  the  Indian, 
Pacific,  Peru,  and 
Georgia  currents, 
and  another 
which  joins  the 
Cape  Horn  cur- 
rent on  its  way 
north  past  the 
Falkland  Islands. 
Soley  (in  various 
reports  issued  by 
the  Hydrographic 
Office  of  the 
United  States 
Navy)  has  dis- 
cussed the  basins 
and  flows  oi  these 
separate  currents 
as  well  as  other 
currents,  and  par- 
ticularly the  Gulf 


THE  MAJOR  CIRCULATIONS  73 

Stream.     He  states  that  the  great  southern  ocean  current 
flows  eastward  around  Antarctica  sometimes  with  a  velocity 
of  two  knots  where  it  narrows  to  pass  through  Drake  Strait. 
In    November  and    December   immense    ice   fields   separate 
from   the  ice  barriers,   starting  generally  as  large  bergs  of 
table  shapes  and  moving  with  the  current;  but       Direction 
as  the  bergs  are  high  out   of  water  the  winds        of  South 
from  the  west   accelerate  their  speed..     In   the        current's 
South    Atlantic    the    direction    of    the    currents        shown  by 
is  shown  very  clearly  by  the  movement  of  the        driftine  we 
ice,  which  travels  north  in  the  Georgia  currents  but  is  cut 
out  very  soon  where  it  meets  the  central  counter  current. 

At  the  north  pole,  conditions  are  different  from  those  at 
the   south  pole.     Here  we  have   a  large,  nearly  landlocked 
water  surface  with  an  average  depth  of  2,000       currents  of 
fathoms.     Bering  Strait  and  the  Greenland  Sea       the  Arctic 
are  the  two  openings  to  the  other  oceans,  and  the 
former  is  only  forty -five  miles  wide  with  a  depth  of  twenty- 
four  fathoms.     The  ocean  is  frozen  nearly  all  the  year.     In 
summer  navigation  is  possible  from  Point  Barrow  east  for  a 
considerable  distance.     A  current  enters  through     The  current 
Bering  Strait  and  then  divides  into  three  branches,     through 
the  polar  current  proper,  and  the  eastern  branch         rmg    trait 
and  the  western  branch.     On  the  other  side  of  the  Arctic 
Ocean  a  great  outgoing  current  is  the  Greenland  current, 
reinforced  by  the  Spitsbergen  current.     There  is  a  current 
flowing   into    the    Arctic    Ocean    known    as    the 
Northeast  current,  which  indeed  is  an  extension  Current 

of  the  Gulf  Stream.     The  warm  Bering  current 
is  in  a  similar  way  an  extension  of  the  great  warm  current 
of  the  Pacific,  namely  the  Kuro   (black)  Shio  (tide),  better 
known  as  the  Japan  current.     The  Greenland  current,  which 
in  a  way  can  thus  be  traced  back  to  the  Bering  current,  joins 
with  the  Labrador  current,  another  branch  of  the 
Bering,    after  .flowing    through    Melville    Sound 
and  Baffin  Bay.     The  Greenland  current  follows 
the  east  coast  of  Greenland  to  Cape  Farewell,  carrying  much 
floe  ice  and  many  large  bergs  from  the  Greenland  glaciers. 
At   Denmark   Strait   we  have  the  strange  spectacle  of  two 


74  THE  PRINCIPLES  OF  ARROGRAPHY 

currents  flowing  in  opposite  directions  within  a  hundred 
miles  of  each  other — the  Greenland  pouring  south  and  the 
Northeast  pouring  north.  On  the  east  coast  of  Greenland, 
temperatures  are  about  272° A.,  while  on  the  Iceland  coast 
the  temperatures  may  reach  284°  A. 

The  great  warm  currents  are  the  north  and  south  equa- 
torials,  the  axes  of  which  are  just  north  of  the  equator  in  the 

Pacific  and  Atlantic  and  south  of  the  equator  in 
currents**  ^e  Indian  Ocean.  The  Pacific  equatorial  recurves 

north  of  the  Philippines  in  the  well-known  Japan 
current,  which  in  its  course  eastward  fans  out  into  what  is 
called  the  "west  wind  drift,"  one  branch  of  this  drift,  how- 
ever, going,  as  we  have  seen,  northward  through  Bering 
Strait.  The  California  current  is  perhaps  the  return  south 
of  the  drift. 

Between  the  California  current  and  the  coast  is  a  north- 
flowing  eddy  current  known  as  the  Davidson  current.  In 
the  Atlantic,  the  Gulf  Stream  is  the  best  known  of  all  ocean 
currents.  Temperature  observations  of  it  were  made  as  early 
as  1775  by  Franklin.  Its  importance  as  a  climatic  control, 

especially  the  widely  accepted  view  that  it  mate- 
Stream1  rially  modifies  the  climate  of  the  British  Isles, 

has  been  somewhat  exaggerated,  the  true  control 
being  the  general  air  movement  over  a  large  water  surface 
from  west  to  east  and  the  high  specific  heat  of  water.  In 
fact,  the  current  which  is  marked  in  the  Gulf  of  Mexico  and 
along  the  Florida  coast  fans  out  and  becomes  a  drift  in  the 
middle  North  Atlantic,  a  portion  of  which,  as  we  have  seen, 
becomes  the  Northeast  current,  washing  Iceland  and  passing 
into  the  Arctic. 

Since  the  time  of  Maury  the  Gulf  Stream  has  been  much 
studied  by  navigators.  Lieutenant  Soley  of  the  United 
States  Navy  (in  charge  of  the  Hydrographic  Office  at  New 

Orleans)  has  contributed  much  to  recent  knowl- 
Chmatic  r  1  •  •  •  •  •  A 

effect  of  edge  of  this  stream,  its  variations  in  intensity, 

Labrador  an(~[  fts  direction.  Probably  the  Labrador  current 
current  .  .  *  *•  1  •  ,  •  rr 

has  a  more   positive  and  direct  climatic  enect 

upon  the  surface  temperature  of  the  eastern  part  of  the 
North  Atlantic  than  is  generally  supposed.  Changes  in  the 


THE  MAJOR  CIRCULATIONS  75 

strength  of  the  trades  in  the  Atlantic  and  the  relation  of 
North  Atlantic  surface  temperatures  to  the  weather  of  the 
British  Isles  have  been  studied  by  Commander  Hepworth, 
R.  N.  R.,  and  the  results  published  by  the  Meteorological 
Office  of  Great  Britain.1  Particularly  valuable  are  the  tables 
of  the  North  Atlantic  mean  temperature  for  the  surface  for 
the  year,  the  monthly  isotherms,  the  thermoisopleths  for 
surface  temperature  from  the  Florida  straits  to  Valencia, 
and  the  charts  of  ice  frequency  and  departures  of  pressure 
and  temperature  for  the  five-year  period  beginning  1908  and 
ending  1912. 

Since  1913  special  cruises  have  been  made  by  officers  of 
the  U.  S.  Revenue  Cutter  Service  with  a  view  to  studying  ice 
conditions  near  the  Grand  Bank  and  on  the 
Labrador  Coast.  The  main  purpose  was  to  conditions  in 
secure  observations  across  the  Labrador  current.  Labrador 
During  the  early  summer  of  1914  ice  conditions 
on  the  Labrador  coast  were  the  worst  for  years  and  vessels 
specially  constructed  to  withstand  heavy  ice  were  unable  to 
make  any  headway.  The  farthest  point  north  reached  by 
the  Seneca  was  52°  45',  longitude  53°  15'.  The  largest  ice- 
berg seen  on  this  cruise  was  nearly  200  meters  long,  180 
meters  wide,  and  15  meters  out  of  the  water,  one  great  block 
of  ice  weighing  about  five  million  tons.  Scattered  about 
were  other  bergs,  the  average  being  about  one  in  every 
five  miles.  South  of  latitude  50°  there  was  a  berg  to  be 
seen  about  every  ten  miles  on  the  average.  It  was  calcu- 
lated that  there  were  about  two  thousand  bergs  between 
Hamilton  Inlet  and  the  Grand  Bank. 

In  future  surveys,  it  may  be  feasible  to  employ  hydro- 
planes, permitting  the  mother  ship  to  remain  at  a  safe 
distance,  while  the  extent  of  the  ice  field  is  being  determined. 
In  foggy  weather  also,  the  plane  can  rise  above  the  fog. 

The  currents  on  the  Grand  Bank  appear  to  be  tidal  and 
largely  influenced  by  the  wind.  The  tide  begins  to  flood  from 
the  southward  and  ends  from  the  northwest;  it  begins  to  ebb 
from  the  northward  and  ends  from  the  southeast,  going  round 
in  almost  a  complete  circle. 

1  Geophysical  Memoirs,  1,  1912,  and  10,  1914. 


76  THE  PRINCIPLES  OF  A&ROGRAPHY 

In  the  opinion  of  Captain  C.  E.  Johnston  of  the  U.  S. 
Revenue  Cutter  Service,  the  results  of  the  two  cruises  made 
Labrador  kv  him  in  these  waters  show  that,  during  June, 
current  the  Labrador  current  dwindles  to  nothing  and 

in  June  ^Q  northern  branch  of  the  Gulf  Stream  spreads 

out,  fan-shaped,  losing  depth  and  temperature  and  becoming 
a  surface  drift,  which  gradually  carries  the  ice  northward  of 
the  Straits  of  Belle  Isle;  there  part  of  the  ice  melts  and  the 
rest  remains  until  the  next  spring,  when  it  comes  south  again 
as  the  Labrador  current  gains  strength. 


CHAPTER  VIII 

THE  MINOR  CIRCULATIONS 

22.  Cyclones  and  anticyclones.  The  word  "cyclone"  was 
introduced  by  Piddington  in  the  first  edition  of  the  Sailor's 
Horn-book  published  in  Calcutta  in  1848.  It  is  from  a  Greek 
word  signifying,  among  other  things,  the  coil  of  a  snake  and  was 
used  as  neither  affirming  that  the  air  moved  in  a  true  circle,  nor 
that  the  circuit  was  complete,  but  still  expressing  the  tendency 
to  circular  motion  in  the  typhoons  of  the  East  Indian  waters. 

Franklin  had  a  clear  idea  of  the  progressive  movement  of 
storms,  as  is  shown  in  his  letter  of  July  16,  1747.     In  another 
letter,  dated  February  13,  1750,  referring  to  an 
eclipse  of  the  moon  on  October  21,  1743,  Franklin      observations 
describes  a   severe  northeast   storm  which  pre- 
vented his   observation   of   the  eclipse   and   his   surprise   in 
finding  that  observations  had  been  made  in   Boston.     He 
says,  "I  wrote  to  my  brother  about  it  and  he  informed  me 
that  the  eclipse  was  over  an  hour  before  the  storm  began." 

Capper  in  1801  described  in  his  Winds  and  Monsoons  certain 
hurricanes  as  revolving  masses  of  air;  and  Horsburgh  in  1805 
described  the  typhoons  of  the  China  Sea  as  rotary  storms.  The 
first  American  storm  to  be  charted  was  that  of  September  23, 
1815,  by  Professor  Farrar  of  Harvard.  '  In  1831  Redfield  pub- 
lished the  first  of  a  series  of  papers  demonstrating  what  was 
then  called  the  Law  of  Storms.  Since  then,  much  has  been 
written  on  the  mechanics  of  cyclones;  and  various  theories 
have  been  advanced  as  to  their  origin,  maintenance,  and 
motion.  Dove,  Piddington,  Reid,  Redfield,  Espy,  Ferrel, 
Hann,  Davis,  Bigelow,  Shaw,  and  others  have  contributed  to 
the  subject ;  but  with  the  introduction  of  new  facts  relating 
to  the  upper  air,  the  various  explanations  have  been  proved 
inadequate.  The  earlier  explanations  assumed  a  movement 
of  the  air  in  circles,  and  an  active  cause  in  the  condensation  of 
vapor  with  uprising  currents;  but  it  now  is  definitely  known 
that  these  are  of  secondary  importance.  By  "cyclone"  we 

77 


78  THE  PRINCIPLES  OF  A&ROGRAPHY 

mean  the  familiar  storm  of  temperate  latitudes,  a  large  aerial 
whirl  or  eddy,  with  a  diameter  of  several  hundred  kilometers. 

_..  In  the  northern  hemisphere  the  rotation  is  in  a 

Direction  of  ^ 

cyclonic  whirl    direction  contrary  to  the  direction  of  the  hands 

of  a  watch,  while  in  the  southern  hemisphere  the 
movement  is  in  the  same  direction  as  the  hands  of  a  watch. 
In  the  center  of  the  whirl  the  pressure  is  lower  than  at  the 
periphery,  and  so  this  is  generally  spoken  of  as  the  storm 
center,  or  depression  center,  although  it  is  by  no  means 
certain  that  this  lowest  pressure,  as  generally  obtained  and 

charted,  is  the  true  center  of  gyration.  Storm 
of  gyration  tracks  plotted  upon  the  assumption  that  the  line 

joining  successive  lowest  pressures  indicates  the 
movement  of  the  center  of  air  motion,  are  misleading,  par- 
ticularly so  when  over-large  corrections  for  surface  tempera- 
ture have  been  used  in  reducing  pressure  readings. 

From  studies  of  storms  in  the  United  States,  Bigelow 
holds  that  there  are  no  true  local  warm-centered  and  cold- 
Mechanism  centered  cyclones  or  anticyclones;  and  that  all 
of  the  lower  theoretical  discussions  founded  on  such  a  basis 
a  mosp  ere  are  misc[irected.  Observations  demonstrate  that 
in  the  lower  atmosphere  the  actual  mechanism  consists  of 
rather  deep,  warm  and  cold  counter-currents  of  air  under- 
running  the  prevailing  eastward  drift.  The  centers  of  gyra- 
tion are  uniformly  in  the  region  where  these  counterflowing 
currents  meet;  that  is,  on  the  edges  rather  than  in  the  midst 
of  the  warm  and  the  cold  regions.  The  front  half  of  the 
cyclone  is  relatively  warm  and  the  other  half  cold;  while  the 
rear  half  of  the  anticyclone  is  warm  and  the  front  half  cold. 
Stratification  and  interpenetration  of  currents  of  different 
temperatures  may  be  the  true  source  of  the  energy  of  storms. 
Source  of  the  The  ^eat  energY  derived  from  the  condensation 
energy  of  of  aqueous  vapor,  and  the  energy  produced  by 

purely  dynamic  eddies,  are  entirely  secondary  in 
importance  to  the  energy  obtained  from  the  counterflow  and 
underflow  of  warm  southerly  currents  against  the  cold  northerly 
currents  and  beneath  the  eastward-flowing  drift.  There 
appears  to  be  no  difference  in  the  structure  of  European 
and  American  cyclones  and  anticyclones.  Instead  of  vertical 


THE  MINOR  CIRCULATIONS 


79 


convection  and  condensation  being  the  prime  movers,  it 
rather  is  a  matter  of  horizontal  convection  and  interchange 
of  heat  on  the  same  general  level.  There  is  no  evidence, 
according  to  Bigelow, 
of  the  superposition  of 
cold-center  cyclones 
upon  warm-center 
cyclones  as  expounded 
by  some  earlier  writers, 
nor  are  there  any  purely 
dynamic  vortices  in  a 
rapid  stream  as  sup- 
posed by  Hann,  nor  are 
there  cyclonic  vortices 
caused  by  atmospheric 
islands  of  high  pressure 
obstructing  a  rapidly 
flowing  eastward  drift. 
The  cyclonic  circu- 
lation constitutes  an 
effort  to  bring  back  to 
equilibrium  the  energy 
difference  represented 
in  cold  and  in  warm 
areas;  and  this  is  done 
by  setting  up  an  exten- 
sive series  of  internal 
vortices  varying  in  size 
from  the  large  storm 
areas  down  through  tor- 
nadoes or  secondaries 
to  the  minute  whirls 
one  sees  in  the  desert 


ooo 


After  Bigelow 

FIG.  24.     TYPICAL  DISTRIBUTION  OF  THE  WIND 
IN  CYCLONES  AND  ANTICYCLONES 


as  dust    whirls    or   on 

the  sea  as  waterspouts. 

In  ordinary  cyclones  the  vortices  are  not  perfect,  and  it  is 

only   rarely  and  in  highly  developed,  localized  storms,  such 

as  the  tornado  and  the  waterspout,    that  continued  vortex 

motion  is  attained.      Discussion  of  these  types  will  be  given 


80 


THE  PRINCIPLES  OF  A&ROGRAPHY 


/oooo 


later.     For  diagrams  of  velocity  and  temperature  in  cyclones 
see  Figs.  24,  25,  and  26. 

W.  H.  Dines,  studying  the  high-level  records,  found  that 

differences  of  pressure 
at  the  earth's  surface 
were  of  the  same  order 
of  magnitude  as  those 
at  a  height  of  9  kilo- 
meters; and  he  drew 
the  inference  that  the 
surface  distribution 
was  mainly  controlled 
by  the  conditions  at 
the  higher  level. 
Shaw  has  carried  this 
view  further  and 
demonstrated  that  the 
intermediate  layers 
contribute  little  to  the 
distribution  at  the 
surface  and  that  the 
stratosphere,  although 
made  up  of  only  one 
quarter  of  the  atmos- 
phere, is  the  dominant 
factor ;  while  the  tropo- 
sphere, though  it  in- 
cludes three  quarters 
of  the  atmosphere,  has 
comparatively  little 
influence.  This  differ- 
ence in  the  influence 
of  the  two  great  layers 
is,  Shaw  thinks,  attrib- 
utable simply  to  the 
characteristic  difference  of  temperature  between  regions  of 
high  and  of  low  pressure.  He  has  deduced  two  equations, 
one  giving  the  increase  of  pressure  difference  in  kilobars 
per  meter  of  height,  and  the  other  for  the  gradient  velocity. 


2000 


ooo 


After  Bigelow 

FIG.  25.     TYPICAL  DISTRIBUTION  OF  THE  TEM- 
PERATURE IN  CYCLONES  AND  ANTICYCLONES 


THE  MINOR  CIRCULATIONS 


81 


Among  other  things,  he  has  proved  that  if  certain  assump- 
tions are  granted  the  distribution  of  pressure  and  temperature 
can  be  computed  from  measurements  of  wind  velocity  at 
certain  heights,  such 
as  are  obtained  from 
the  observations  of  a 
pilot  balloon. 

A  number  of  cases 
are  discussed  where 
the  slope  of  the  tem- 
perature is  in  different 
directions,  also  cases  of 
inversion  and  compar- 
ison made  with  types 
of  wind  structure  as 
given  by  Cave.  What 
is  called  the  operative 
distribution  of  pres- 
sure, at  a  height  of 
about  9  kilometers  or 
the  level  at  which  the 
wind  velocity  is 
greatest,  is  used  in  con- 
nection with  * '  Egnell '  s  " 
or  Clayton's  law,1  that 
is,  the  law  of  equal 
mass  transport  at  all 
heights,  or  wind  veloc- 
ity increasing  as 
density  decreases,  to 
show  that  up  to  the 
highest  cloud  level 
the  amount  of  air  car- 
ried by  each  kilometer  FIG.  26. 
layer  is  the  same.  It 
would  appear,  then,  from  the  pilot-balloon  soundings,  that 
the  maintenance  of  the  pressure  gradient  and  consequent 

1  This  law  was  first  demonstrated  in  connection  with  the  cloud  observations 
at  Blue  Hill  Observatory  by  Clayton. 


ooo 


After  Bigelow 

TYPICAL  DISTRIBUTION  OF  THE  PRESSURE 
IN  CYCLONES  AND  ANTICYCLONES 


82  THE  PRINCIPLES  OF  A&ROGRAPHY 

increase  of  velocity  aloft  are  really  due  to  the  influence  of 
the  thermal  structure  of  the  underlying  atmosphere  upon  the 
pressure  distribution  transmitted  from  above.  The  most 
general  characteristic  of  that  structure  is  a  slope  of  tem- 
perature toward  the  north.  A  general  statement  that  the 
velocity  always  increases  aloft  would  certainly  be  subject  to 
G  .  many  exceptions,  but  it  seems  correct  to  say 

characteristic     that  the  usual  thermal  structure  of  the  atmos- 

of  the  thermal  phere  is  such  that  the  westerly  component  of  the 
structure  ^ .  .  .  ,  . 

air  current  is  greatest  in  the  operative  pressure 

layer  and  gradually  diminishes  from  there  to  the  surface. 

Shaw1  gives  five  laws  of  atmospheric  motion  which  seem  to 
him  to  be  fundamental.     These  are: 

1.  The  law  of  the  relation  of  motion  to  pressure.     In  the  upper  layers 
of  the  atmosphere  the  steady  horizontal  motion  of  the  air  at  any 
level  is  along  the  horizontal  section  of  the  isobaric  surfaces  at 
that  level,   and  the  velocity  is  inversely  proportional  to  the 
separation  of  the  isobaric  lines  in  the  level  of  the  section. 

This  law  is  not  experimentally  demonstrated  and  it  is 
quite  possible  it  may  be  of  limited  application.  It  presup- 
Five  laws  of  poses  a  condition  of  steady  motion.  Shaw  points 
atmospheric  out  that  allowance  must  be  made  for  "curva- 
ture of  path"  which  will  vary  with  latitude.  For 
half  of  the  globe  north  of  30°  N.  and  south  of  30°  S.  it  is  generally 
negligible,  but  near  the  equator  it  becomes  the  paramount 
consideration  in  the  question  of  the  persistence  of  distribution. 

"Thus,  rotary  systems,  small  or  large,  are  the  only  possible 
isobars  for  a  synchronous  chart  of  an  equatorial  region,  if  one 
were  drawn.  Long  sweeps  of  parallel  isobars  would  be  impossible 
there." 

2.  The  law  of  the  computation  of  pressure  and  of  the  application  of 
the  laws  of  gases. 

This  is  simply  an  amplification  of  the  well-known  equation 
of  elasticity. 

3.  The  law  of  convection.     Convection  in  the  atmosphere  is  the  descent 
of  colder  air  in  contiguity  with  air  relatively  warmer. 

1  Principia  Atmospherica. 


THE  MINOR  CIRCULATIONS  83 

4.  The  law  of  the  limit  of  convection.     Convection  in  the  atmosphere 
is  limited  to  that  portion  of  it  called  the  troposphere,  in  which 
there  exists  a  sensible  fall  of  temperature  with  height.     The 
upper  layer  of  the  atmosphere,  in  which  there  is  no  sensible  fall 
of  temperature  with  height  and  therefore  no  convection,  is  called 
the  stratosphere. 

5.  The  law  of  saturation.     The  amount  of  water  vapor  contained  in 
a  given  volume  of  space  cannot  exceed  a  certain  limit  which 
depends  upon  the  temperature. 

As  a  postulate,  based  on  Dines'  statistical  studies,  it  may  be 
stated  that  in  the  stratosphere  from  11  kilometers  upward,  it 
is  colder  in  the  high  than  in  the  low  at  the  same  level;  and  in 
the  troposphere,  from  9  kilometers  down  to  1  kilometer,  it  is 
warmer  in  the  high  pressure  than  in  the  low  at  the  same  level. 
The  average  horizontal  circulation  in  the  northern  hemisphere 
in  January  between  4  kilometers  and  8  kilometers  consists  of  a 
figure-of-eight  orbit  from  west  to  east  along  isobars  around 
the  pole,  with  lobes  over  the  continents  and  bights  over  the 
oceans.  The  average  circulation  at  the  surface  is  the  resultant 
of  the  circulation  at  4  kilometers  combined  with  a  circulation 
in  the  opposite  direction  of  similar  shape  due  to  the  distribution 
of  temperature  near  the  surface. 

Shaw  in  a  recent  lecture1  on  "Illusions  of  the  Upper  Air," 
using  what  he  has  called  the  principle  of  strophic 
balance,  shows  that  this  has  the  great  advantage      balance°P 
of  giving  a  definite  relation  between  wind  veloc- 
ity, pressure,  and  temperature.     Using  the  familiar  symbols 

p  for  pressure 

9  for  temperature 

p  for  density 

/  for  horizontal  distance 

h  for  vertical  height 

5  for  horizontal  pressure  gradient 

q  for  horizontal  temperature  gradient 

v  for  velocity  of  wind,  positive  when  pressure  is  high  on  the 

right  of  the  path 

E  for  the  radius  of  the  earth 

g  for  normal  acceleration  of  gravity 

r  for  the  angular  radius  of  a  small  circle  on  the  earth's  surface 

which  indicates  the  path  of  air  in  a  cyclone 

1  Lecture  before  Royal  Institution,  March  10,  1916;  Nature,  April  27,  1916, 
p.  191,  and  May  4,  1916,  p.  210. 


84  THE  PRINCIPLES  OF  A&ROGRAPHY 

• 

<f>    for  latitude 

co    for  the  angular  velocity  of  the  earth's  rotation 

he  obtains  for  the  fundamental  relation  between  the  velocity 
of  the  wind  at  any  level  and  the  pressure  gradient  there 

--  =  s  =  2  covp  sin  </>  ±  v2  p  cot.  r. 


The  two  terms  which  make  up  the  right-hand  side  of  this 
equation  are  of  different  importance  in  different  places  and 
circumstances.  If  the  air  is  moving  in  a  great  circle,  r  is  90°, 
and  cot.  r  is  zero;  so  that  the  first  term  alone  remains.  At 
the  equator  the  latitude  is  zero  and  the  second  term  alone 
remains.  Away  from  the  equatorial  region  the  second  term 
is  relatively  unimportant  unless  the  velocity  is  great.  In 
temperate  and  polar  latitudes  the  path  of  the  air  differs 
little  from  a  great  circle  except  in  rare  cases  near  the  center 
of  deep  depressions;  consequently  the  first  term  may  be  re- 
garded as  the  dominant  term  in  these  regions. 

Shaw  calls  the  wind  computed  from  the  first  term  the 
geostropkic  wind  or  practically  the  actual  wind  of  temperate 
Geostrophic  and  polar  region.  The  wind  computed  according 
wmd  to  the  second  term  is  called  the  cyclostropkic  wind 

Cyclostrophic  or  the  actual  wind  in  equatorial  regions,  or  the 
wind  of  tropical  hurricanes. 

By  simple  manipulation  of  the  fundamental  formulae  in- 
cluding the  characteristic  equation  Shaw  obtains  the  following  : 

For  change  of  pressure  gradient  with  (  ds_  _         /  q  ___£\ 

height  \dh      gP\B      p) 

For  change  of  wind  velocity  with  height  :    (  dv_  _  vdd   __  g_  m  q 
geostrophic  winds  .  \  dh~  Odh      2  co  sin  4>    6 

(  dv2       M6         gE       q 

cyclostropmc  winds  -j  —  =  T-JT-  H  —     —* 

(  dh       Bdh       cot.  r    6 

From  these  equations  he  deduces  what  may  be  called  the 
law  of  equivalence  of  pressure  distribution  and  wind  which 
in  his  opinion  also  serves  to  explain  the  following  facts  estab- 
lished by  observation: 

Light  winds  in  the  central  region  of  an  anticyclone:  It 
follows  from  the  fundamental  equation  when  the  negative 


THE  MINOR  CIRCULATIONS  85 

sign  is  taken,  as  it  must  be  for  an  anticyclone,  that  the  values 
of  v  will  be  given  by  the  roots  of  a  quadratic  equation,  which 


Ett)  sin</> 
will  be  impossible  if  v  is  greater  than     CQt  r    '     This  for  a 

radius  of  113  kilometers  (70  miles)  allows  a  velocity  of  only 
about  4  meters  per  second.  This,  Shaw  holds,  furnishes  a 
crucial  test,  for  if  an  anticyclone  is  a  region  of  descending  and 
outward  flowing  air,  the  velocity  should  diminish  as  the  air 
spreads  outward.  In  an  anticyclone  the  reverse  condition 
occurs.  This,  however,  is  open  to  the  criticism  that  in  prac- 
tice advancing  cyclones  may  mask  the  true  circulation  and 
velocity. 

The  small  influence  of  the  troposphere,  and  therefore  the 
dominance  of  the  stratosphere  in  the  distribution  of  surface 
pressure,  follow  in  Shaw's  opinion  when  numerical  values  are 
given  in  the  equation  for  change  of  pressure  gradient  with 
height  or  js 

dh  = 

The  right-hand  side  of  the  equation  consists  of  two  terms 
which  are  of  opposite  sign  and  numerically  nearly  equal  in 
the  middle  regions  of  the  troposphere.  Their  combined 
effect  for  the  whole  range  is  therefore  relatively  small  and  the 
change  of  pressure  produced  in  the  troposphere  is  unimportant. 
The  distribution  of  the  stratosphere  is  dominant  throughout 
the  troposphere. 

Again,  the  apparently  capricious  variations  of  wind  and 
temperature  with  height  shown  by  pilot  balloons  and  ballons- 
sondes  may  be  connected  numerically  by  the  equation  for 
the  change  of  pressure  gradient  with  height  and  the  equation 
for  horizontal  pressure  gradient.  Shaw  gives  an  example 
where  the  rapid  transition  from  a  southerly  wind  at  1,100 
meters  through  a  calm  to  a  northerly  wind  at  1,500  meters  on 
October  16,  1913,  indicated  a  temperature  gradient  of  7° 
per  hundred  kilometers  toward  the  east.  But  this  condition 
was  in  satisfactory  accord  with  the  meteorological  circum- 
stances at  the  time.  The  same  combination  of  equations 
enables  us  to  specify  the  conditions  under  which  "EgnelVs 
law"  that  wind  velocity  at  different  heights  is  inversely 


86  THE  PRINCIPLES  OF  ARROGRAPHY 

proportional  to  the  density  at  those  heights,  may  be  expected 
to  be  verified,  and  the  conditions  prescribed  are  essentially 
reasonable. 

The  rapid  falling  off  of  wind  in  the  stratosphere  noted  in 
observations  with  pilot  balloons  follows  from  the  equation 
for  change  of  wind  velocity  with  height. 

The  same  equation  applied  to  the  troposphere,  assuming 
normal  values  for  temperature,  gives  correctly  the  rate  of 
change  of  velocity  with  height.  The  permanence  of  vertical 
motion  about  a  vertical  axis  in  the  atmosphere  follows  from 
the  equation  for  cyclostrophic  winds.  With  a  wind  velocity 
of  20  meters  per  second  and  a  horizontal  temperature  gradient 
of  5°  per  hundred  kilometers,  the  extension  will  be  1.4  kilo- 
meters upward;  so  that  the  vortex  will  be  covered  by  a  cap 
in  which  the  velocity  gradually  falls  off  to  zero  within  a  very 
limited  height. 

For  the  extension  downward  the  calculation  is  more  com- 
plicated; but  the  computed  change  of  velocity  is  very  small 
so  that  the  vortex  must  be  regarded  as  reaching  the  ground; 
and  it  would  appear  that  a  vortex  extending  throughout  the 
troposphere  terminating  with  a  cap  in  the  stratosphere  is  a 
possible  reality. 

Thus  the  hypothesis  of  an  atmosphere  in  which  the  wind 
velocity  is  everywhere  adjusted  to  balance  the  pressure  dis- 
tribution, enables  us  to  explain  many  of  the  ascertained  facts 
that  have  been  disclosed  by  the  investigation  of  the  upper  air, 
and  strongly  supports  the  idea  that  the  pressure  distribution 
at  the  surface  is  controlled  by  the  stratosphere  and  only 
modified  locally  by  convection. 


CHAPTER  IX 


FORECASTING  STORMS 

23.  Types  of  storms.  It  is  the  practice  of  the  Forecast 
Division  of  the  Weather  Bureau  to  classify  storms  after  the 
region  where  first  charted.  Thus,  one  of  the  most  frequent 
types  is  known  as  the  " Alberta,"  because  it  is  definitely 
charted  in  that  territory.  The  system  has  its  defects,  and 
with  each  enlargement  of  the  area  of  observation 
some  modification  of  the  place  of  origin  becomes 
apparent.  In  a  recent  publication  by  Bowie 
and  Weightman1  the  types  are  given  as  Alberta, 
North  Pacific,  South  Pacific,  Northern  Rocky 
Mountain,  Colorado,  Texas,  East  Gulf,  South  Atlantic, 
Central,  and  West  Indian.  Charts  showing  the  normal 
twenty-four-hour  movement  for  five-degree  squares  have  been 
prepared  to  take  the  place  of  the  earlier  charts  of  paths  of 
greatest  frequency.  Tables  covering  a  period  of  twenty  years 
are  also  available  for  the  average  velocity.  Thus  it  is  seen 
that  storms  of  continental  types  move  more  rapidly  in  winter 
than  in  summer. 


Type  of 
storm 
named  for 
place  of 
origin 


Kilometers 
in  24  Hours 

Miles 

January  
February  
March  
April 

1,199 
1,110 

1,080 

871 

745 
690 
673 
542 

May 

792 

492 

June  
July.  . 

772 
839 

480 
521 

August  
September  

788 
883 

489 
549 

October  
November  
December  

919 
1,040 
1,156 

571 
646 

718 

Average  for  year. 

954 

593 

Monthly  Weather  Review,  Suppl.  1,  July,  1914;  also  Suppl.  4,  Jan.,  1917. 

87 


88  THE  PRINCIPLES  OF  A&ROGRAPHY 

In  determining  a  possible  deviation  from  a  normal  course, 

account  is  taken  of  unequal  pressure  distribution  in  the  regions 

.  adjacent  to  the  storm  center,   also  the  location 

rules  for  of  maximum  twelve-hour  pressure  fall   and   the 

movement         trend  of  the  isotherms.     A  number  of  empirical 

rules  based  upon  the  intensity    and    movement 

of  the  twelve-hour  pressure  change  are  given  by  Bowie  for 

the  movement  of  lows. 

The  most  important  rules1  for  the  guidance  of  the  fore- 
caster in  determining  the  course  of  a  hurricane  are: 

"A  hurricane  does  not  move  directly  toward  a  region  of 
high  pressure  when  such  an  area  is  not  moving  perceptibly, 
but  follows  behind  it.  If  the  high  moves  east 
hurricane?11  or  northeast  off  to  sea  at  a  normal  rate,  the 
hurricane  moves  north  or  northeast.  If  the 
high  hangs  persistently  over  the  coast,  the  hurricane  is  de- 
flected far  to  the  west  before  it  can  recurve. 

"If  rain  falls  freely  before  the  hurricane  comes  to  land,  the 
disturbance  may  decrease  in  intensity;  but  if  heavy  rain 
begins  after  the  storm  passes  inland,  the  storm  will  probably 
continue. 

"When  a  West  Indian  hurricane  is  moving  westward  in  the 
longitude  of  eastern  Cuba  and  is  north  of  that  island,  it  will 
recurve  east  of  the  South  Atlantic  coast,  when  an  area  of  high 
pressure  covers  the  northwestern  states.  If  the  hurricane  is 
moving  westward  over  Cuba  or  the  western  Caribbean  Sea 
when  a  low  area  occupies  the  northwest,  and  the  pressure  is 
high  in  the  eastern  states,  the  storm  will  probably  move  to  the 
Gulf  of  Mexico  and  reach  the  Gulf  coast  after  recurving. 

"For  storms  over  the  Great  Lakes,  it  appears  that  depres- 
Storms  over  sions  frequently  remain  stationary  or  move 
the  Great  slowly,  accompanied  with  much  precipitation, 
Lakes  when  the  pressure  is  high  to  the  north  and  north- 

east. Again,  the  movement  is  slow  when  the  air  from  an 
extensive  high-pressure  area  drains  southeast  from  the 
Missouri  Valley. 

' '  Other  storms  that  increase  with  intensity  appear  to  depend 

1  In  Weather  Forecasting  in  the  United  States  many  general  statements  are 
made  by  the  various  forecasters  regarding  the  movements  of  HIGHS  and  Lows. 


FORECASTING  STORMS  89 

on  marked  horizontal  temperature  gradients.  A  rapid  tem- 
perature rise  in  front  of  a  storm  implies  an  increase  in  intensity, 
especially  if  the  temperature  is  falling  rapidly  over  the  north- 
west. Sharp  temperature  rises  in  the  eastern  quadrants  of  a 
storm  are  a  sure  indication  that  the  storm  will  move  north- 
eastward and  increase  in  intensity." 

On  the  Atlantic  coast  there  are  certain  types  of  disturbance 
which,  especially  in  March,1  have  provoked  widespread  com- 
ment  owing   to   the   failure   of   the   Washington 
forecasters  rightly  to  anticipate  weather  condi-  storms 

tions  of  the  succeeding  twenty-four  hours.     Note- 
worthy instances  were  those  at  the  time  of  the  presidential 
inaugurations  in  1897  and  1909. 

For  forty-five  years  the  forecasters  at  Washington  and  for 
shorter  periods  at  other  forecast  centers  such  as  San  Francisco, 
Chicago,  New  Orleans,  Portland,  and  Denver 
have  depended  mainly  in  making  their  forecasts 
upon  certain  auxiliary  maps  of  pressure  and 
temperature  changes.  Attempts  have  been  made  to  use 
cloud  change  and  humidity  change  maps,  but  for  reasons 
hardly  satisfactory,  it  would  seem,  no  continuous  use  of  these 
latter  charts  has  been  made.  The  pressure  and  temperature 
auxiliary  maps  show  the  24-  and  12-hour  changes.  The 
barometric  tendency,  as  defined  by  the  Inter-  The 
national  Committee  in  1913,  is  the  change  in  the  barometric 
three  hours  preceding  the  observation.  This  is  ency 

not  used  in  the  United  States,  but  whenever  the  pressure  has 
risen  or  fallen  1.02  millimeters  within  two  hours  preceding  the 
observation,  the  change  is  reported,  though  not  the  character 
of  the  change,  such  as  steady,  unsteady,  etc. 

The  pressure  chart  gives  the  area  and  intensity  of  the  non- 
periodic  pressure  changes.  Henry,  compiling  the  fluctuations 
in  pressure  at  certain  stations  for  a  period  of  ten  years,  found 
that  the  frequency  was  nearly  the  same  for  all  parts  of  the 
country  except  that  the  changes  are  more  rapid  at  northern 
stations.  The  average  annual  number  of  such  pressure 
movements  is  88  and  the  average  time  interval  4.2  days. 

!The  snow  and  wind  storm  known  as  the  Great  Blizzard  of  New  York 
occurred  March  12-14,  1888. 


90  THE  PRINCIPLES  OF  ARROGRAPHY 

Ekholm  in  1911  suggested  the  terms  allobar  for  the  area  of 
pressure  change ;  anallobar  for  an  area  over  which  the  pressure 
has  risen ;  and  katallobar  for  an  area  over  which  the  pressure 

has  fallen  within  the  given  time.     The  region  of 
Allobars  .  ,  fe,  ,    ,  . 

maximum  change  may  be  regarded  as  the  center. 

The  names  are  cumbersome  and  the  conditions  might  well 
be  described  simply  as  "rises"  and  "falls."  Henry1  has 
described  the  basis  of  forecasting  by  synoptic  maps,  and  given 
at  some  length  the  relation  of  the  pressure  change  areas  and 
the  movements  of  highs  and  lows.  Shaw2  has  discussed 
certain  relations  between  the  isallobars  and  the  winds. 
Storms  over  the  Lake  region  sometimes  develop  secondaries 

off  the  Virginia  or  New  Jersey  coasts;  and  these 
Secondaries  s    ^       1       1  ^  -,  •        < 

pass  apparently  slowly  northward,  causing  heavy 

snows  and  high  winds  in  the  Middle  Atlantic  and  New  England 
states. 

Of  all  secondaries,  tornadoes  are  most  destructive  and 
most  frequent.  They  are  associated  with  storms  of  increasing 
energy,  moving  to  the  left  of  normal  paths  when  the  trough 
of  low  pressure  extends  well  southward.  (For  a  good  illus- 
tration see  the  weather  map  of  April  29,  1909.)  Again,  when 
the  southern  portion  of  the  trough  swings  eastward  faster 
than  the  northern  portion,  there  is  likelihood  of  the  develop- 
ment of  a  secondary  storm  south  or  southeast  of  the  northern 
center.  (See  map  of  November  8,  1913.) 

There  is  a  tendency  for  secondaries  to  form  to  the  leeward 
of  the  Appalachian  Mountains,  following  the  passage  east- 
ward of  moderate  disturbances  from  the  northwest.  A 
pressure  rise  coming  from  the  Lake  region  seems  to  play  an 
important  part.  If  this  moves  south  of  the  low,  secondaries 
do  not  develop. 

For  apparent  paths  of  lows  and  highs  in  the  United  States 
as  given  by  Van  Cleef  see  Figs.  27  and  28. 

24.  Tornadoes.  These  are  secondary  and  localized  vortices 
of  great  energy,  forming  as  a  rule  in  the  southern  quadrants 
of  a  larger  cyclonic  storm.  In  the  southern  hemisphere 
this  relation  is  reversed.  The  characteristic  feature  is  a 

1  Weather  Forecasting  in  the  United  States,  p.  69. 

2  Forecasting  Weather,  pp.  337-341. 


FORECASTING  STORMS 


91 


Paths  miscellaneous 57 

Paths  incomplete 120 

Total  paths 

Scale— 

5  paths  =  5  mm. 
Less  than  25  paths  =  single  11 


FIG.  27.     STORM  PATHS  IN  THE  UNITED  STATES  After  Van  cieef 


Paths  classified 741 

Paths  miscellaneous 

Paths  incomplete * 

Total  paths 
Scale— 

123  paths  =  5  mm. 
Less  than  25  paths  = 


FIG.  28.     PATHS  OF  HIGHS  IN  THE  UNITED  STATES      After  Van  Cleef 


92  THE  PRINCIPLES  OF  AEROGRAPHY 

funnel-shaped  cloud,  although  the  term  is  widely  used  for 
any  marked  wind  storm  of  great  violence.  The  word  was 
originally  used  for  certain  storms  off  the  African 
coast  m  which  there  was  a  quick  change  of  wind 
direction.  The  word  is  used  in  the  press  as  equiv- 
alent to  "twister,"  and  often  wrongly  used  as  the  equivalent 
of  cyclone.  Tornadoes  are  best  denned  as  extremely  violent 
vortices  with  funnel-shaped  clouds,  the  diameters  of  which 
vary  from  100  to  500  meters,  while  the  larger  storms  (cyclones) 
of  which  they  are  a  part  have  diameters  a  hundred  times 
greater.  Owing  to  centrifugal  force,  the  pressure  is  very  low 
Cause  of  within  a  limited  area;  and  as  ordinarily  stated, 

their  destruc-  a  partial  vacuum  is  developed.  The  destructive 
effects  therefore  are  due  both  to  the  extremely 
high  velocity  of  the  wind,  which  may  reach  100  meters  per 
second,  and  to  the  rapid  change  in  pressure.  The  vortex  moves 
east  or  northeast  at  a  rate  of  10  or  15  meters  per  second  and 
may  preserve  its  identity  for  an  hour  or  longer,  and  it  is  not 
unusual  for  several  tornadoes  to  form  within  a  few  kilometers 
of  each  other  and  move  in  parallel  paths.  These  storms 
are  very  destructive,  and  phenomena  that  seem  almost 
beyond  belief  occur  during  their  passage  over  a  given  spot. 
Effects  of  Thus  chickens  have  been  stripped  of  their 
tornadic  feathers,  large  animals  have  been  lifted  and 

activity  carried   some   distance,    straws,   twigs,  and  other 

small  articles  have  been  driven  into  boards,  owing  to  their 
high  velocity,  notwithstanding  their  small  mass.  The  walls 
of  buildings  fall  outward  as  in  an  explosion,  and  trees  are 
uprooted,  falling  as  the  whirling  air  leaves  them.  Thus  on 
the  north  side  of  the  path  of  the  center,  trees  are  thrown  to 
the  southeast,  while  on  the  south  side  they  are  thrown  to  the 
northeast.  Tornadoes  occur  under  the  same  general  con- 
ditions as  thunderstorms,  along  the  line  of  conflict  between 
warm,  moist  south  winds  and  cold  north  winds.  They  are 
frequent  in  the  spring.1  Grinnell,  Iowa,  was  devastated 
by  a  tornado  on  June  17,  1882,  and  Rochester,  Minn.,  on 
August  21,  1883.  Tornadoes  of  historic  interest  occurred  at 

!For  good  examples  of  tornado  conditions,  see  the  weather  maps  of  Feb. 
19,  1884,  and  Mar.  23,  1913. 


FORECASTING  STORMS 


93 


Lawrence,    Mass.,    July    26,    1890    (Fig.   29);    at    St.    Louis, 
May  27,  1896;  and  at  Omaha  and  various  points  in  Nebraska 
and  adjoining  states  on  March  23,  1913.     Finley        vortices  of 
has  discussed  statistically  this  type  of  storm  for        certain 
the  United  States.     The  storm  as  an  example  of        tornad°es 
vortex  motion  has  been  studied  by  Bigelow,  who  has  computed 
the  radial  and  tangential  velocities  at  various  points  for  the 
St.   Louis  tornado  of  May  27,   1896. l     Briefly,   the  vortices 
extended  upward  about  1,200  meters  and  had  a  diameter  of 


FIG.   29.     TORNADO  AT  LAWRENCE,   MASS. 

The  center  of  the  tornado  passed  along  the  side  of  the  street  nearest  the 
overturned  house.  The  house  just  beyond  it  was  drawn  forward  six  or  eight 
feet  to  the  edge  of  the  sidewalk  shown  by  the  small  tree. 

1,900  meters  near  the  ground.  The  velocity  of  the  air  near 
the  center  of  the  vortex  at  the  ground  exceeded  250  meters 
per  second. 

25.  Waterspouts.  These  are  small  secondary  storms 
occurring  generally  at  sea,  although  there  are  instances  of 
their  occurrence  inland  over  water  courses.  Thus  on  July 
16,  1904,  a  well-defined  spout  was  observed  by  many  people 
over  the  Hudson  River  near  Tarry  town.  This  is  described 
by  M.  L.  Bacon,2  who  succeeded  in  getting  six  photographs 

1  Monthly  Weather  Review,  Aug.,  1908. 
*Ibid.,  June,  1906. 


94 


THE  PRINCIPLES  OF  AEROGRAPHY 


of  the  spout.  Apparently  the  cloud  did  not  descend  in  a 
funnel  shape  to  meet  the  rising  column  of  water;  but  the 
column  of  water  rose,  clear  cut,  and  met  the  cloud,  forming 
a  vertical  column  at  4:25  P.M.  At  4:35  P.M.  the  spout  had 
disappeared. 

A  waterspout  which  occurred  off  Cottage  City  in  Vineyard 
Sound  on  August  19,  1896,  has  been  studied  in  detail  by 
Conditions  Bigelow,1  who  came  to  the  conclusion  that  it 
preceding  was  caused  by  a  sheet  of  cold  air  overrunning 
the  low,  warm,  quiet  strata,  about  midday; 
while  the  cold  air  followed  at  the  surface  a  few  hours  later. 
In  such  facts  we  have  the  conditions  required  to  produce 
marked  vertical  convection, — sudden  cooling  of  the  upper 
strata  and  an  abnormal  stratification. 

The  following  description  of  a  triple  waterspout  is  given  by 
F.  A.  D.  Cox,  lieutenant  in  the  Royal  Navy:2 

"On  November  10,  1912,  about  10  A.M.  .  .  .  bright 
sunshine,  when  gradually  the  sky  became  overcast  and  there 


Photograph  by  Chamberlain 

FIG.  30.     THE  GREAT  WATERSPOUT  EIGHT  MILES  OFF  COTTAGE  CITY,  MASS., 

AUGUST  19,   1896 

was  a  dense  rain  cloud  above  as  shown  in  the  photograph. 
Below  the  bank  of  clouds  to  sea  level  it  was  quite  clear  and 

1  Monthly  Weather  Review,  July,  1906. 
2Symons's  Meteor.  Magazine,  April,  1913, 


FORECASTING  STORMS 


95 


F.  A.  D.  Cox  in  Symons's  Meteor.  Magazine 

FIG.  31.     WATERSPOUTS  AT  CHATHAM  ISLANDS 


bright  and  toward 
the  east  heavy  rain 
was  evidently  fall- 
ing. A  little  after 
11,  my  attention 
was  called  to  a 
peculiar  funnel- 
shaped  cloud  which 
was  beginning  to 
appear  on  the 
lower  edge  of  the 
cloud  bank.  I  saw 
at  once  that  some- 
thing out  of  the 
common  was 

beginning  and  said:  'Well,  I  have  never  seen  a  waterspout, 
but  it  looks  to  me  as  if  this  was  one  forming.'  We  then 
carefully  watched  and  it  was  soon  plainly  evident 
that  a  waterspout  was  taking  place.  We  marked  waterspout 
how  the  funnel-shaped  excrescence  from  the  cloud 
bank  gradually  extended  downward  to  the  sea,  and  from 
below  we  could  observe  another  funnel  rising  which  soon 
joined  the  one  above;  the  whole  appearance  had  that  of  a 
spiral  tube  evidently  formed  by  a  rotary  motion ;  the  water  on 
the  sea  end  of  the  spout  was  in  a  perfect  foam.  The  spout 
first  formed  was  to  the  right  hand  on  the  eastward  side  next 
to  where  the  heavy  rain  was  evidently  falling.  Almost  imme- 
diately after  the  formation  of  the  first  spout  another  began 
to  make  its  appearance.  It  was  much  thicker  than  the  first, 
and  as  in  that  case  the  sea  below  the  cloud  was  violently 
agitated,  and  even  from  where  we  stood,  which  must  have 
been  seven  or  eight  miles  distant,  the  form,  like  columns,  was 
plainly  seen.  This  was  by  far  the  largest  of  the  spouts  and 
continued  for  nearly  half  an  hour.  Toward  the  end,  before 
the  spout  began  to  subside,  the  sea  had  almost  the  appearance 
of  a  geyser,  so  violently  was  it  agitated.  I  think  the  large 
one  must  have  been  nearer  to  us  than  the  first,  for  as  the 
first  began  to  dissolve,  it  gradually  drifted  toward  the  big 
one,  and  soon  the  remains  appeared  as  a  sort  of  appendix 


96  THE  PRINCIPLES  OF  AEROGRAPHY 

hanging  from  the  cloud  above.  The  spouts  disappeared 
slowly,  and  the  whole  phenomenon  occupied  about  three 
quarters  of  an  hour." 

Thunderstorms  are  also  secondary  storms;  but  owing  to 
the  marked  electrical  phenomena  accompanying  them,  the 
discussion  is  more  properly  placed  under  the  section  on  atmos- 
pheric electricity. 


CHAPTER   X 
THE  WINDS 

26.  Wind  systems.     Edmund  Halley,   the  astronomer,   in 
1698  received  from  King  William  III  command  of  a  sailing 

ship,  with  directions  to  study  variations  of  the 

TT  -,  ,     First  knowl- 

compass.     He  made  two  memorable  voyages  and    edge  of  wind 

practically  covered  the  Atlantic  from  50°  N.  to 
50°  S.  Besides  the  magnetic  work  much  meteor- 
ological work  was  done,  and  our  first  knowledge  of  the 
general  wind  system  of  the  Atlantic  comes  as  a  result  of 
these  voyages.1 

In  1856  another  explorer,  Captain  Charles  Wilkes  of  the 
United  States  Navy,  read  a  paper  before  the  American  Asso- 
ciation for  the  Advancement  of  Science  which  was  later  pub- 
lished. It  is  in  this  work  that  we  find  included  one  of  the 
earliest  maps  of  the  winds  of  the  world.  In  1875,  Early  maps 
through  the  joint  agency  of  the  Smithsonian  In-  and  charts 
stitution  and  Professor  J.  H.  Coffin  of  Lafayette  of  winds 
College,  there  was  published  a  large  volume  on  The  Winds  of 
the  Globe.  Several  world  maps  of  wind  movement  are  given; 
and  not  only  the  annual  direction  but  the  directions  for  summer 
and  winter  months  are  charted.  Koppen  of  Hamburg  has 
given  us  the  latest  of  these  charts. 

It  is  apparent  that  there  are  several  mighty  streams  of 
air  flowing  around  the  world  in  certain  latitudes.  Some 
blow  steadily  and  are  more  or  less  permanent, 
like  the  trades,  the  anti-trades,  and  the  pre-  *fa 
vailing  westerlies.  Some  are  seasonal  in  char- 
acter, like  the  monsoons.  There  are  also  well-marked  minor 
circulations,  known  as  sea  breezes  and  valley  winds;  and 
finally,  there  are  the  individual,  localized  winds  accompany- 
ing the  various  storm  types. 

The  permanent,  or  planetary,  winds  are  controlled  by  the 
planetary  pressure  distribution;  that  is,  they  depend  upon  the 

1  See  section  18. 

8  97 


98  THE  PRINCIPLES  OF  ARROGRAPHY 

general  difference  of  temperature  between  equatorial  and  polar 
regions,  and  more  especially  upon  the  position  and  strength 
Agencies  °^  ^he  §rea^  planetary  pressure  belts.  The  sea- 

controlling  sonal  winds  can  be  correlated  with  movements  of 
the  winds  ^  hype^^g  or  infrabars  (the  so-called  centers 
of  action).  The  local  winds  can  be  traced  to  temporary 
disturbances  of  pressure. 

The  winds  have  been  classified  by  Dove,  Davis,  and  others 
as  planetary,  terrestrial,  continental,  land  and  sea  breeze, 
mountain  and  valley  breeze,  cyclonic,  and  certain  accidental 
winds  due  to  volcanic  eruptions. 

27.  Trade  winds.  These  are  the  great  northeast  and 
southeast  wind  systems.  The  name  is  derived  from  the 
old  English,  to  blow  trade,  meaning  in  one  direction. 
On  the  pilot  charts  issued  by  the  Hydrographic  Office 
founded  upon  the  researches  made  and  the  data  collected 
by  Lieutenant  M.  F.  Maury,  there  is  pub- 
ofPtheSPacffic  lisne(i  tne  average  condition  of  wind  and  weather 
for  the  given  period.  Thus  if  we  look  on  the 
chart  of  the  North  Pacific  Ocean  for  May,  we  find  that  the 
northeast  trades,  force  4  to  5  (5  to  8  meters  per  second), 
extend  to  within  about  5°  of  the  American  coast  between 
the  25th  and  15th  parallels.  They  average  twenty-four  days 
in  May  over  the  Hawaiian  Islands.  On  the  other  hand,  the 
southeast  trades,  force  3  to  4  (3  to  5  m.  p.  s.),  extend  1°  to  5° 
north  of  the  equator,  and  are  farthest  north  between  longitudes 
150°  W.  and  110°  W. 

Over  the  Atlantic  during  May  we  find  that  the  northeast 

trades  extend  northward  slightly  beyond  the  Canary  Islands, 

but  west  of  the  30th  meridian  the  northern  limit 

of  these  winds  is  nearly  along  the  25th  ParalleL 
The  southern  limit  is  close  to  the  equator  on  the 

American  side,  but  rises  to  latitude  12°  N.  at  longitude  20°  W. 
The  force  of  the  northeast  trades  is  4  to  5,  increasing  toward 
the  south.  Their  direction  is  northerly  off  the  African  coast, 
but  is  northeast  between  the  20th  and  30th  meridians. 
Farther  westward  the  direction  is  more  easterly,  and  north 
of  the  Lesser  Antilles  it  is  southeasterly,  showing  the  anti- 
cyclonic  circulation  around  the  Azores  hyperbar.  The  winds 


THE  WINDS  99 

are  generally  east  to  northeast  in  the  Caribbean  Sea  and  east 
to  southeast  in  the  Gulf  of  Mexico.  The  southeast  trades, 
force  3  to  4,  extend  from  1°  to  30°  above  the  equator  between 
the  8th  and  42d  meridians. 

Over  the  Indian  Ocean  during  May  the  southeast  trades 
prevail  over  the  area  between  the  equator  and  latitude  30°  S. 
Over  the  extreme  northern  and  southern  portions  Spring  trades 
of  this  area  the  trades  are  broken  by  variable  of  the  Indian 
winds,  and  calms  are  frequent  between  the  Ocean 
equator  and  10°  S.  The  trades  are  steadiest  between  lati- 
tudes 10° S.  and  25°  S.  Along  the  African  coast,  near  the 
equator,  they  follow  the  contour  of  the  land  and  merge  into 
the  southwest  monsoon. 

If  we  follow  the  trades  during  winter  months  we  shall  find 
that  over  the  North  Pacific  the  northeast  trades  reach  their 
most  northern  limit  in  the  eastern  part  of  the 
ocean  at  the  29th  parallel,  slightly  southeast  of 
the  central  area  of  the  California  high,  and  are 
strongest  and  steadiest  south  of  this,  region.  Between  longi- 
tudes 145°  W.  and  155°E.  their  northern  limit  is  close  to  the 
25th  parallel.  They  extend  eastward  to  within  5°  to  8°  of 
the  American  coast  and  westward  to  Asiatic  waters,  where 
they  merge  into  the  northeast  monsoon.  They  extend  as 
far  south  as  the  equator  west  of  longitude  170°E.  East  of 
this  longitude  their  southern  limit  gradually  rises  to  the  10th 
parallel  at  longitude  125°W.  The  southeast  trades  extend 
north  of  the  equator  between  longitudes  85°  W.  and  180°  W. 
They  reach  their  most  northern  limit,  the  6th  parallel,  between 
longitudes  115°W.  and  125°  W.  In  .the  North 
Atlantic  the  northeast  trades  prevail  between 
the  5th  and  25th  parallels.  Near  Brazil  they  ex- 
tend as  far  south  as  the  equator;  and  near  the  African  coast 
as  far  north  as  latitude  32°  N.  These  winds  are  the  typical 
northeast  trades  over  the  eastern  part  of  the  ocean  and  in 
the  Caribbean  Sea.  In  the  central  part  of  the  ocean  they 
become  east-northeasterly.  Southeast  trade  winds  extend 
north  of  the  equator  over  the  central  part  of  the  ocean  to 
the  4th  parallel. 

In  the  South  Atlantic  the  southeast  trades  prevail  from  the 


100  THE  PRINCIPLES  OF  ARROGRAPHY 

area  of  high  pressure  to  latitude  5°S.  on  the  eastern  part  of 
the  ocean,  and  from  latitude  15°  S.  to  the  equator  on  the 
western.  Over  the  greater  part  of  this  area  they  are  well 
developed,  blowing  from  the  southeast  from  50  to  60  per 
cent  of  the  time,  with  a  small  percentage  of  calms  and  no 
gales,  the  average  force  being  about  4.  South  of  the  area  of 
high  pressure,  "the  brave  west  winds"  prevail.  They  have 
increased  slightly  in  intensity  since  the  spring.  The  winds 
around  the  high  show  their  anticyclonic  movements  very 
Winter  trades  plamly>  while  those  within  the  area  are  variable 
of  the  Indian  in  direction  and  force.  Over  the  Indian  Ocean 

in  winter  the  southeast  trades  occur  between 
10°  S.  and  30°  S.  east  of  the  50th  meridian.  Their  average 
force  is  3  to  4.  West  of  Madagascar  the  winds  are  mostly 
northeasterly  and  southeasterly,  while  south  of  Madagascar 
they  are  easterly. 

In    discussing   the   planetary    circulation   it    was    assumed 
that  in  the  upper  levels  there  must  be  an  overflow  of  air  from 

the  equator  to  the  poles.  Recent  soundings, 
Anti-trades  however,  do  not  confirm  this  view.  And  this 
counter-trades  variability  in  flow  is  sjgown  in  marked  degree 

above  the  trades.  The  trades  themselves  are 
comparatively  shallow  streams,  not  extending  above  the 
5-kilometer  level.  Above  these  the  air  movement  is  from 
west  to  east;  and  these  winds  are  called  the  anti-trades,  some- 
what unfortunately  since  this  term  is  applied  to  the  winds 
farther  north  or  south,  better  described  as  the  prevailing 
westerlies.  We  shall  use  the  term  ' '  counter- trades "  for  the 
winds  above  the  trades.  The  counter-trades,  then,  are  above 
the  trades  and  extend  approximately  from  4  to  16  kilometers, 
or  more  than  twice  the  depth  of  the  surface  trades.  Still 
higher  and  above  the  counter-trades  flows  the  so-called  upper 

easterly  current,  extending  up  to  a  height  of 
easterlies  ^0  kilometers,  and  above  this  again,  a  westerly 

flow  in  the  same  direction  as  the  counter-trades, 
and  approximately  5  kilometers  in  depth.  Finally  at  a 
height  of  30  kilometers  there  would  seem  to  be  another  easterly 
current.  Thus  over  the  tropics  we  find  at  least  five  wind 
systems.  As  we  move  to  higher  latitudes,  the  winds,  possibly 


THE  WINDS    - 


101 


under  the  influence   of  the  deflective  tendency  due  to  the 
earth's   rotation,    change   their   direction   through   the   south 
and  become  eventually  west  winds.     These  air  streams  are 
drier  and  heavier  than  the  trades,  and  descend  to  the  surface 
in  latitude  30°  from  the  thermal  equator,  as  warm,  dry  south- 
west   winds.     In    the    United    States,    the    anti- 
trades are  more  commonly  called  the  prevailing         westerlies 
westerlies,  winds  which  lack  the  steadiness  of  the 
trades,  but  which  nevertheless  are  the  controlling  factors  in 
determining  the  weather  of  the  temperate  zones. 

In  connection  with  the  movement  of  the  upper  air  it  is  of 
interest  to  note  that  the  dust  from  the  Krakatau  eruption 
in  1883,  a  few  degrees  south  of  the  equator,  was  carried  from 
east  to  west  around  the  world  in  about  15  days.  The  red 
sunsets1  and  sunrises  due  to  the  fine  dust  and  vapor  particles 
appeared  progressively  later  from  west  to  east  and  indicated 
an  average  movement  of  113  kilometers  per  hour,  31  m/s. 

28.  Monsoons.  The  word  monsoon2  is  said  to  be  of 
Arabic  origin,  meaning  "season,"  and  rightly  applies  to 
the  winds  of  the  Indian  Ocean,  for  the  general  character  of 
the  season  and  the  crop  yield  are  closely  connected  with  the 

duration  and  intensity  of  these  winds.     During 

,1  j_i        j_i  ji  The  summer 

the    summer    months    the    southwest    monsoon,      m0nsoons 

force  3  to  5  (3  to  8  m.  p.  s.),  dominates  the  ocean 

north  of  the  equator.     It  overspreads  the  Arabian  Sea  early 

in  June,  and  by  the  third  week  is  in  full  force  over  the  Bay  of 

Bengal.     Severe  thunderstorms,   thick,   cloudy  weather,   and 

gales  with  occasional  dangerous   cyclones   occur   during   the 

period  immediately  preceding  the  full  force  of  the 

monsoon.     In  winter  we  have  to  deal  with  the        monsoons* 

northeast  monsoon,  force  3  to  4  (3  to  5  m.  p.  s.), 

which   prevails   over   Indian   waters   and   extends   down   the 

African  coast  to  latitude  10°  S.     Northwesterly  winds  prevail 

in  the  Persian  Gulf  and  the  Gulf  of  Oman,  and  easterly  winds 

in  the  Gulf  of  Aden.     In  the  southern  part  of  the  Red  Sea 

the  winds  are  southeasterly,  and  in  the  northern  part  they  are 

1  See  end  of  section  44,  "Why  sunsets  are  red." 

2  Alexander  the  Great  is  said  to  have  brought  back  to  Greece,  after  his  invasion 
of  India,  information  concerning  the  monsoon. 


102  THE  PRINCIPLES  OF  ARROGRAPHY 

northwesterly.  On  the  Asiatic  coast  the  winds  east  of  Chosen 
(Korea)  are  northeasterly;  west  of  it  they  are  northwesterly. 
Along  the  China  coast  immediately  north  of  Shanghai  to  the 
5th  parallel  they  are  northeasterly  and  are  known  as  the 
northeast  (winter)  monsoon.  The  monsoon  is  in  full  force 
during  January,  and  blows  with  greatest  strength  and  con- 
stancy between  Macao  and  Chusan.  It  shows  a  marked 
tendency  to  follow  the  coast,  and  as  it  weakens  at  night  and 
the  wind  becomes  somewhat  offshore,  northbound  sailing 
vessels  may  then  make  fair  headway.  The  thick,  rainy 
weather  of  the  monsoon  period  renders  navigation  difficult 
off  the  coast  of  Taiwan  (Formosa) .  A  rising  pressure  foreruns 
an  increase  in  the  strength  of  the  monsoon,  and  a  falling 
pressure  a  decrease. 

29.  Local  winds.  In  nearly  every  land  there  are  local 
names  for  special  winds,  based  as  a  rule  upon  the  warm  or 
cold,  and  wet  or  dry  character  of  the  wind.  In  mountainous 
countries,  especially  if  the  range  is  but  a  short  distance  from 
some  large  water  surface,  the  air  at  times  seems  to  rush 
through  the  valleys  and  canons.  This  can  nearly  always  be 
traced  to  the  passage  of  some  general  disturbance.  There  is 
another  class  of  day-and-night  winds  which  are  due  primarily 
Winds  to  differences  of  temperature  in  the  valley  and  at 

influenced  by  the  level  of  the  mountain  tops,  also  sometimes 
topography  to  differences  in  the  heating  of  the  east  and  the 

west  sides  of  the  range.  These  are  the  well-known  mountain 
and  valley  winds,  reversing  their  direction  with  the  change 
from  night  to  day.  In  all  these  wind  systems  the  contour  of 
the  land  plays  an  important  part.  Study  of  the  topography 
shows  that  the  drafts  are  localized  and  intensified  by  the  lay 
of  the  land.  Most  of  these  winds  are  in  the  nature  of  forced 
drafts,  in  the  sense  that  air  masses,  generally  with  moderate 
momentum,  forced  through  restricted  channels,  such  as 
mountain  passes  and  valleys,  are  drafts.  These  are  chiefly 
horizontal  currents,  while  the  regular  mountain  and  valley 
winds  are  more  often  due  to  vertical  currents. 

It  is  not  surprising,  therefore,  to  find  that  the  direction  of 
the  flow  may  be  determined  to  some  degree  by  the  trend  of 
the  narrow  air  passage  or  valley.  Thus,  although  the  foehn 


THE  WINDS  103 

wind  in  the  Alps  is  primarily  a  south  or  southwest  wind, 
it  may  appear  in  certain  districts  as  a  southeast  wind, 
having  had  its  direction  of  flow  deflected  by  the  Forced  drafts 
trend  of  the  valley.  Furthermore,  displacement  of  mountain 
at  one  place  means  motion  at  some  other  point  pass( 
of  the  circuit,  and  we  may  have  an  endless  chain,  as  it  were,  in 
which  the  natural  flow  is  masked.  And  just  as  in  the  case  of 
the  flow  of  water  in  rivers  there  may  be  established  return 
currents  at  the  sides  of  the  main  stream,  or  eddy  currents  at 
points  where  obstruction  to  the  general  flow  is  met,  in  the 
central  part  of  a  valley  the  flow  of  air  or  wind  may  be  in  one 
direction,  while  on  the  sides  the  flow  may  be  in  an  opposite 
direction.  An  excellent  way  of  studying  the  flow  of  air  in 
mountainous  countries  is  from  a  station  on  the  summit. 
Close  observation  of  the  clouds  above  and  the  fogs  below,  as 
they  form  and  dissipate,  will  show  the  existence  of  many 
unsuspected  air  streams. 

Of  all  the  special  winds,   the  foehn  is  perhaps  the  best 
known  as  it  has  been  most  studied  and  written  about.     The 
word  is  of  German  origin  and  possibly  is  con- 
nected with  the  Latin  favonius,  a  west  wind  of         wind°e 
the  spring;  but  if  such  were  the  original  meaning 
it  is  not  in  accord  with  the  conditions  now  existing,  for  the 
foehn    is  essentially   a    dry   south   wind.     It    blows  on    the 
northern  slopes  of  the  Alps  and  is  most  noticeable  in  those 
valleys   which   have   a  north-and-south   trend;   indeed,   it   is 
hardly  noticeable  in  some  valleys  which  extend  east  and  west. 
For  many  years  it  was  explained  as  originating  in 
the  deserts  of  Africa;  but  it  is  now  known  to  be  origin0 

the  southerly  component  of  an  indraft  due  to  the 
passage  of  a  cyclonic  area  over  Western  Europe.     The  word 
foehn  is  also  used  in  a  broader  sense  to  designate  any  wind 
system  where  the  air,  moving  into  a  cyclone,  is  forced  over 
some  range  and  thus  cooled  and  dried;  and  then,  descending 
on    the   farther    slopes,    is    dynamically    heated. 
Under  such  conditions  evaporation  is  rapid  and      typ®  Qfw^ 
snow  on  the  ground  disappears  quickly.     In  the 
Northern  Hemisphere  such  winds  in  temperate  latitudes  are 
generally  from  the  south,  and  in  the  Southern  Hemisphere 


104  THE  PRINCIPLES  OF  A&ROGRAPHY 

from  the  north.  Thus  we  find  foehn  winds  in  Greenland, 
Iceland,  Eastern  Europe  as  in  Hungary  (rotenturmwind) ,  South 
America,  Japan,  Peru,  and  in  fact  in  all  parts  of  the  world 
where  mountains  act  as  partial  barriers  to  the  flow  of  air 
and  there  is  compression  after  expansion.  Such  a  wind  is  the 
The  chinook  c^no0^  (name  of  an  Indian  tribe  dwelling  on 

Puget  Sound),  a  dry  and  relatively  warm  wind 
of  Wyoming,  Montana,  Idaho,  eastern  Oregon,  and  parts  of 
Colorado.  A  good  illustration  of  the  pressure  distribution 
and  resulting  wind  direction  and  temperature  can  be  found 
on  the  weather  map  for  January  23,  1907.  The  temperature 
rose  quickly  from  about  255°A.  to  275°A.  and,  as  previously 
stated,  the  snow  evaporated  rapidly.  The  duration  of  the  wind 
depends  upon  the  movement  of  the  low-pressure  area  to  the 
"Hot  winds,"  north.  Sometimes  the  high  temperature  will  last 
^'northers,"  twenty-four  hours.  Other  warm,  dry  winds  are 

the  so-called  "hot  winds"  of  the  Plains  states,  the 
summer  winds  of  Texas,  the  "northers"  of  the  Sacramento 
and  San  Joaquin  valleys,  and  the  "Santa  Anas"  of  southern 
California.  Some  of  these  winds  in  the  summer  months  pass 
over  heated  areas  and  are  warmed  to  some  degree  by  radiation 
Sirocco  from  the  earth.  The  sirocco  of  southern  Italy 

and  Greece  is  a  warm  south  wind,  generally  dust- 
laden  and  therefore  trying  to  man  and  beast.  The  leveche  of 
Spain  and  the  leste  of  the  Madeira  Islands  are  sirocco  winds; 
the  solano  is  an  east  wind  on  the  east  coast  of  Spain;  the 
harmattan  is  a  hot,  dusty  east  wind  of  the  winter  months  in 

the  Gulf  of  Guinea;  the  simoon  (from  the  Arabic 
and  simoon  word  for  poison,  although  there  are  no  poisonous 

gases  associated  with  it)  is  a  hot,  sand-laden  wind 
felt  in  Palestine,  Syria,  and  Arabia;  the  khamsin  of  Egypt  is 
a  hot  southeast  wind  which  blows  for  about  fifty  days  after 
the  middle  of  March;  the  brickfielders  are  hot  north  winds 
of  Southern  Australia.  There  are  many  others  having  local 
names. 

30.  Cold  waves  and  boreal  winds.  The  word  boreal,  from 
Boreas,  the  north  wind  of  the  Greeks,  is  used  to  designate 
a  class  of  cold  winds  generally  of  cyclonic  origin.  There 
would  seem  to  be  some  connection  between  the  intensity  of 


THE  WINDS  105 

the  depression  and  the  temperature  of  the  northwest  quadrant. 
Being  preceded  by  comparatively  warm  southerly  winds,  the 
contrast  is  marked  and  all  the  more  noticeable.  The  air  is 
not  necessarily  brought  from  high  levels,  and  the  compression 
is  not  sufficiently  great  to  warm  the  air  enough  to  affect  mate- 
rially its  initial  low  temperature.  The  so-called  cold  waves  of 
the  United  States  are  essentially  boreal  winds.  Such,  too,  are 
the  buran  or  purga  of  Russia,  the  pamperos  of  Argentina,  the 
southerly  burster  of  New  Zealand,  the  bora  of  the  Adriatic, 
and  the  mistral  of  the  valley  of  the  Rhone.  The  word 
"mistral"  is  derived  from  the  Latin  magister,  and  the  wind  is 
therefore  appropriately  described  as  a  master  wind,  or  the  wind 
which  dominates.  It  has  been  known  for  some  time  that  the 
winds  of  the  Antarctic  region  were  of  higher  velocity  and  lower 
temperature  than  elsewhere,  and  the  records  of  the  Australian 
Antarctic  Expedition  of  1911-1914  confirm  this.  Thus  gusts 
of  wind  having  a  velocity  of  nearly  90  meters  per  second 
(200  miles  per  hour)  were  recorded  on  the  Robinson  anemom- 
eters used.  The  record  is  subject  to  correction,  and  these 
figures  may  be  reduced  20  per  cent.  Velocities  of  80  meters 
per  second  and  even  higher  were  not  infrequent,  nor  were 
winds  of  45  meters  per  second  (100  miles  per  hour)  with  a 
temperature  of  240°A.  (-28°F.)  rare. 

31.  Charts  for  aeronauts  and  aviators.  The  term 
"aeronaut"  is  used  to  designate  the  pilot  of  a  balloon,  while 
"aviator"  is  restricted  to  the  pilot  of  a  heavier-than-air  flying 
machine.  One  of  the  first  attempts  to  bring  the  results  of 
the  exploration  of  the  air  by  kites,  balloons,  and  other  means 
into  convenient  form  for  the  use  of  aviators  and  aeronauts  is 
the  volume  by  Rotch  and  Palmer,1  issued  in  1911.  Charts  of 
the  relative  heights,  corresponding  densities,  and  temperatures 
are  given.  The  frequency  of  winds  from  various  directions 
and  their  respective  velocities  at  Blue  Hill  are 

shown.     Thus  the  shallowness  of  easterly  winds      Shallowness 

.  •;.,,.,  and  depth 

is    made    evident    by    comparisons    at    different      Of  winds 

levels.     The  summer  sea  breeze  has  a  depth  of 

about    1,000    meters,    while   the   easterly   winds   of    cyclonic 

origin  may  have  a  depth  of  2,000  meters.     The  winds  of  winter 

1  Charts  of  the  Atmosphere  for  Aeronauts  and  Aviators. 


106  THE  PRINCIPLES  OF  AEROGRAPHY 

are  of  greater  velocity  than  the  winds  of  summer.  In  brief 
west  winds  are  most  frequent.  Near  the  ground  they 
blow  about  25  per  cent  of  the  time  from  south-southwest 
to  west-southwest  in  summer,  and  about  33  per  cent  of 
the  time  from  west  to  northwest  in  winter,  with  a  velocity 
varying  from  8  to  11  meters  a  second.  At  a  height  of  3,000 
Summer  and  meters  the  frequency  of  the  westerly  winds 

winter  increases  to  nearly  twice  that  at  the  lower  level, 

variations  1    -1  •  -i  •  1 

and  there  is  a  corresponding  increase  in  velocity. 

Particular  attention  is  paid  to  the  problem  of  trans-Atlantic 
flight  and  the  possibility  of  utilizing  the  northeast  trade  for 
The  trades  the  western  voyage  is  considered.  The  height  at 
and  trans-  which  the  southwest  counter  trade  may  be  ex- 
itic  flight  pected  is  uncertain.  It  has  sometimes  been 
found  below  1,500  meters,  and  again  has  been  absent  at 
10,000  meters. 

Another  excellent  book  is  that  of  C.  J.  P.  Cave,  entitled  The 
Structure  of  the  Atmosphere  in  Clear  Weather,  which  gives  the 

result  of  two  hundred  observations  of  pilot  bal- 
models  loons  and  ballons-sondes.  The  direction  and 

velocity  of  the  wind  at  different  levels  are 
charted.  Cardboard  models  show  the  distribution  of  wind 
with  each  kilometer  of  height.  The  arrowhead  flies  with 
the  wind.  The  gradient  wind  is  computed  from  the  distri- 
bution of  pressure.  The  velocity  is  calculated  from  the 
measured  distance  of  two  isobars  between  which  the  station 
lies  by  means  of  the  formula 


sn 


where  G  is  the  gradient,  co  the  angular  velocity  of  the  earth, 
0  the  latitude,  V  the  velocity,  and  p  the  density  of  the  air. 

Five  types  of  atmospheric  structure  are  described:  (1)  where 
the  wind  in  the  upper  air  is  steady  and  there  is  no  in- 
Five  types  of  crease  of  velocity  with  height  ;  (2)  where  the 
atmospheric  wind  is  steady  but  increases  in  velocity  much 
above  the  gradient  value;  (3)  where  the  upper 
wind  decreases  in  velocity;  (4)  where  changes  of  direction 
or  reversals  occur  ;  and  (5)  where  the  upper  air  blows  away 
from  centers  of  low  pressure. 


THE  WINDS  107 

The  strongest  current  is,  as  a  rule,  just  below  the  strato- 
sphere ;  and  in  view  of  the  work  of  Dines  and  the  suggestions 

of  Shaw,  the  question  is  raised  whether  it  would 

.  .  Strength  of 

not  be  advantageous  to  refer  variations  in  the        air  current 

different  levels  to  the  conditions  in  the  9-kilometer  j^1^  with 
level  instead  of  to  the  surface.  Starting  with  a 
strong  westerly  wind  under  the  stratosphere,  Cave  would 
then  work  downward,  for  almost  without  exception  the  west 
wind  decreases  in  the  lower  levels  and  the  falling  off  may 
proceed  continuously  to  such  an  extent  that  the  direction  of 
motion  is  reversed  at  some  point  in  the  intermediate  layers, 
so  that  near  the  surface  an  easterly  wind  is  shown  instead  of 
the  westerly  one  of  the  upper  levels. 

The  term  "holes  in  the  air"  has  been  used  by  aviators  to 
describe  certain  unstable  conditions  experienced  when  flying 
and  which  in  their  opinion  are  caused  either  So-called 
by  "holes"  in  the  air,  or  by  partial  vacuums  or  "holes  in 
"pockets."  Such  conditions  are  found  on  sum- 
mer mornings  and  afternoons  and  near  cumulus  clouds,  and 
are  generally  simply  ascending  currents  of  some  momentum 
through  which  the  flyer  passes.  Sometimes  the  aviator  may 
skirt  such  a  column  of  uprising  air  (and  there  are  Ascending 
also  descending  currents)  and  part  of  the  plane  and  descend- 
be  within  the  current  while  the  rest  may  be  mg  cum 
without.  In  such  cases  there  will  be  sudden  tilting  or  inequal- 
ity of  pressure,  and  the  aviator  should  be  careful  not  to  attempt 
to  meet  the  changed  conditions  too  quickly,  for  they  are  but 
temporary  and  instability  will  result  when  the  machine  is 
again  free.  Not  only  near  cumuli  but  also  near  cross  currents, 
-  that  is,  where  one  air  stream  is  flowing  in  a  different  direc- 
tion from  an  adjacent  stream, —  there  will  be  minor  vortices 
and  more  or  less  of  an  air  surge;  and  such  a  condition  will 
cause  instability  of  the  aeroplane.  As  has  been  explained  in 
preceding  paragraphs,  there  is  sometimes  a  marked  stratifica- 
tion of  the  lower  air,  and  under  certain  conditions  marked 
turbulence.  When  such  conditions  are  suspected  it  might 
be  advisable  to  resort  to  preliminary  tests  by  freeing  pilot 
balloons  or  pilot  planes. 


112  THE  PRINCIPLES  OF  ARROGRAPHY 

but  few  of  them  have  proposed  entirely  new  systems,  and 
nearly  all  have  been  content  to  modify  slightly  the  types 
proposed  by  Howard.  The  list  includes  Poey,  Forster,  Clos, 
Kaemtz,  Fritsche,  Jevons,  Clouston,  Muhry,  Ley,  Weilbach, 
Vettin,  Klein,  Koppen,  Tissandier,  Barker,  Moller,  Toynbee, 
Jesse,  Abercromby,  Hildebrandsson,  Maze,  Singer,  Neumayer, 
Kassner,  Riggenbach,  Akerblom,  de  Bort,  Fitzroy,  Clayden, 
Clayton,  Gaster,  Vincent,  and  many  others.1 

A  remarkable  cloud  record  is  that  of  S.  C.  Russell,  extending 
over  eight  years.  He  accumulated  in  the  course  of  this  period 
nearly  a  hundred  thousand  observations.2 

In    1894    Clement    Ley,    in   his   book   entitled 
classification      Cloudland,  proposed  a  new  classification  consisting 

of  four  main  divisions:  (1)  radiation  clouds,  (2) 
interfret  clouds,  (3)  inversion  clouds,  (4)  inclination  clouds. 

Under  the  first  division  are  all  the  fog  types;  under  the 
second,  clouds  caused  by  the  interaction  of  horizontal  currents; 

under  the  third,  the  cumulus  types,  or  clouds 
dSfication  caused  by  condensation  due  to  convectional 

currents;  and  under  the  fourth,  the  cirrus  types. 
Clayton,  in  1889,  suggested  the  following  classification3  based 
upon  the  origin  of  the  cloud: 

1.  Clouds  due  to  local,  nearly  vertical,  ascending  currents.     In  this 
group  belong  the  forms  known  as  cumuli. 

2.  Clouds  due  to  slow  and  oblique  ascending  currents.     In  this 
group  belong  the  sheet  clouds  of  stratification. 

3.  Clouds  due  to  the  chilling  of  the  lower  air  by  radiation  from  the 
earth.     In  this  class  belong  the  fogs. 

4.  Clouds  due  to  the  evaporation  of  the  thinner  parts  of  clouds 
already  formed,   probably  caused   by  descent.     In   this   group 
belong  many  of  the  clouds  which  appear  in  "flocks"  of  balls  or 
rolls;  also  certain  forms  of  cirrus. 

5.  Clouds  due  to  differences  in  the  direction  and  velocity  of  air 
currents  at  different  levels.     In  this  class  belong  the  cirrus. 

1  Goethe  was  much  interested  in  meteorological  phenomena,  especially  in 
cloud  forms.     He  carried  on  a  correspondence  with  Howard,  which  extended 
over  many  years.     An  interesting  statemsnt  of  Goethe's  meteorological  views, 
together  with  nine  sketches  of  cloud  types,  is   given    in  W.  v.  Wasielewski's 
Goeihes  meteor  ologische  Studien,  Leipzig,   1910. 

2  Quart.  Jour,  of  the  Royal  Met.  Soc.,  Oct.,  1913.     The  clouds  are  grouped 
in  four  main   classes:     (1)   upper  clouds;    (2)   intermediate  clouds;    (3)   lower 
clouds,  including  fog;  and  (4)  diurnal  ascending  current  formations. 

„      3  Annals  Harvard  Observatory,  Vol.  XXX,  Part  IV,  1896. 


WATER  VAPOR  OF  ATMOSPHERE 


113 


About  1890,  chiefly  through  the  efforts  of  Hildebrandsson, 
an  international  cloud  classification  was  agreed  upon,  and  in 


FIG.  35.     MORNING  FOG  RISING. 


McAdie 

SEA  FOG  AUGMENTED  BY  RADIATION  FOG 


1896  there  appeared  an  international  cloud  atlas,  by  Hilde- 
brandsson, Riggenbach,  and  de  Bort. 

35.  The  international  system.  During  the  summer  of 
1894  a  committee,  which  had  been  appointed  by  the  Inter- 
national Meteorological  Congress  at  Munich,  met  at  Upsala  to 
prepare  an  atlas  representing  the  cloud  forms  with  the  nomen- 
clature proposed  by  Hildebrandsson  and  Abercromby,  and 
recommended  for  general  use  by  the  congress.  The  definitions 
adopted  by  this  committee  are  as  follows: 

1.  Cirrus  (Ci.).     Isolated  feathery  clouds  of  fine  fibrous  texture, 

generally  of  a  white  color,  frequently  arranged  in  bands  which 
spread  like  the  meridians  on  a  celestial  globe  over  a  part  of 
the  sky  and  converge  in  perspective  toward  one  or  two  opposite 
points  of  the  horizon.  In  the  formation  of  such  bands  Ci.S.  and 
Ci.Cu.  often  take  part. 

2.  Cirro-stratus  (Ci.S.).     Fine  whitish  veil,  sometimes  quite  diffuse, 

giving  a  whitish  appearance  to  the  sky,  and  called  by  many 
"cirrus  haze,"  sometimes  of  more  or  less  distinct  structure, 
exhibiting  tangled  fibers.  The  veil  often  produces  halos  around 
the  sun  and  moon. 

9 


FIG.  36.     STRATO-CUMULUS.     Low  FOG 


FIG.  37.    CLOUD  CHANGES.    ALTO-CUMULI 
114 


McAdi 


WATER  VAPOR  OF  ATMOSPHERE  115 

3.  Cirro-cumulus   (Ci.Cu.).     Fleecy   cloud.     Small  white  balls  and 

wisps  without  shadows,  or  with  very  faint  shadows,  which  are 
arranged  in  groups  and  often  in  rows. 

4.  Alto-cumulus  (A.Cu.).     Dense  fleecy  cloud.     Larger  whitish  or 

grayish  balls  with  shaded  portions  grouped  in  flocks  or  rows, 
frequently  so  close  together  that  their  edges  meet.  The  dif- 
ferent balls  are  generally  larger  and  more  compact  (passing 
into  S.Cu.)  toward  the  center  of  the  group,  and  more  delicate 
and  wispy  (passing  into  Ci.Cu.)  on  its  edges.  They  are  very 
frequently  arranged  in  lines  in  one  or  two  directions. 

The  term  "  cumulo-cirrus"    is   given   up   because   it   causes 
confusion. 

5.  Alto-stratus  (A.S.).     Thick  veil  of  a  gray  or  bluish  color,  exhibiting 

in  the  vicinity  of  the  sun  and  moon  a  brighter  portion,  which, 
without  causing  halos,  may  produce  coronas.  This  form  shows 
gradual  transitions  to  cirro-stratus,  but  according  to  the 
measurements  made  at  Upsala  it  .has  only  half  the  altitude. 

The  term  "stratus-cirrus"  is  abandoned  because  it  gives  rise 
to  confusion. 

6.  Strato-cumulus  (S.Cu.).     Large  balls  or  rolls  of  dark  cloud  which 

frequently  cover  the  whole  sky,  especially  in  winter,  and  give 
it  at  times  an  undulated  appearance.  The  stratum  of  strato- 
cumulus  is  usually  not  very  thick,  and  blue  sky  often  appears 
in  the  breaks  through  it.  Between  this  form  and  the  alto- 
cumulus all  possible  gradations  are  found.  It  is  distinguished 
from  nimbus  by  the  ball-like  or  rolled  form,  and  because  it 
does  not  tend  to  bring  rain. 

7.  Nimbus   (N.).     Rain   clouds.     Dense  masses  of  dark,   formless 

clouds  with  ragged  edges,  from  which  generally  continuous  rain 
or  snow  is  falling.  Through  the  breaks  in  these  clouds  is  almost 
always  seen  a  high  sheet  of  cirro-stratus  or  alto-stratus.  If  the 
mass  of  nimbus  is  torn  up  into  small  patches,  or  if  low  fragments 
of  cloud  are  floating  much  below  a  great  nimbus,  they  may  be 
called  "fracto-nimbus,"  the  "scud"  of  the, sailors. 

8.  Cumulus  (Cu.).     Wool-pack  clouds.     Thick  clouds  whose  summits 

are  domes  with  protuberances,  but  whose  bases  are  flat.  These 
clouds  appear  to  form  in  a  diurnal  ascensional  movement,  which 
is  almost  always  apparent.  When  the  cloud  is  opposite  the 
sun,  the  surfaces  which  are  usually  seen  by  the  observer  are 
more  brilliant  than  the  edges  of  the  protuberances.  When 
the  illumination  comes  from  the  side,  this  cloud  shows  a  strong 
actual  shadow ;  on  the  sunny  side  of  the  sky,  however,  it  appears 
dark  with  bright  edges.  The  true  cumulus  shows  a  sharp 
border  above  and  below.  It  is  often  torn  by  strong  winds,  and 
the  detached  parts  present  continual  changes  ("fracto- 
cumulus"). 


FIG.  38.     CLOUD  CHANGES.     ALTO-STRATUS 


FIG.  39.     CLOUD  CHANGES.     ALTO-STRATUS 
Figs.  38  and  39  were  taken  at  an  interval  of  one  minute. 

116 


WATER  VAPOR  OF  ATMOSPHERE  117 

9.  Cumulo-nimbus  (Cu.N.).     Thunder  cloud;  shower  cloud.     Heavy 

masses  of  clouds,  rising  like  mountains,  towers,  or  anvils, 
generally  surrounded  at  the  top  by  a  veil  or  screen  of  fibrous 
texture  ("false  cirrus")  and  below  by  nimbus-like  masses  of 
cloud.  From  their  base  generally  fall  local  showers  of  rain  or 
snow,  and  sometimes  hail  or  sleet.  The  upper  edges  are  either 
of  compact  cumulus-like  outline,  and  form  massive  summits, 
surrounded  by  delicate  false  cirrus,  or  the  edges  themselves  are 
drawn  out  into  cirrus-like  filaments.  This  last  form  is  most 
common  in  "spring  showers."  The  front  of  thunderstorm 
clouds  of  wide  extent  sometimes  shows  a  great  arch  stretching 
across  a  portion  of  the  sky,  which  is  uniformly  lighter  in  color. 

10.  Stratus   (S.).     Lifted  fog  in  a  horizontal  stratum.     When  this 

stratum  is  torn  by  the  wind  or  by  mountain  summits  into 
irregular  fragments,  the  clouds  may  be  called  "fracto-stratus." 

The  committee  also  adopted  the  following  instructions  for 
recording  clouds: 

"  1.  The  kind  of  cloud  designated  by  the  international  letters  of  the 
cloud  name,  which  may  be  more  exactly  defined  by  giving  the 
number  of  the  picture  in  the  atlas  most  nearly  representing 
the  observed  form. 

"2.  The  direction  from  which  the  clouds  come.  If  the  observer  remains 
completely  at  rest  during  a  few  seconds,  the  motion  of  the 
clouds  may  easily  be  studied  by  noting  their  relative  position 
to  a  steeple  or  other  tall  object,  such  as  a  mast,  in  an  open  space. 
"If  the  motion  of  the  cloud  is  very  slow,  for  such  an  obser- 
vation one's  head  must  be  supported.  Clouds  should  be 
observed  in  this  way  only  near  the  zenith;  for  if  they  are  too 
far  away  from  it,  the  perspective  may  cause  errors.  In  this 
case,  nephoscopes  should  be  used,  and  the  rules  followed  which 
apply  to  the  particular  instrument  employed. 

"3.  Radiant  point  of  the  upper  clouds.  These  clouds  often  appear  in 
the  form  of  fine  parallel  bands,  which  by  an  effect  of  perspective 
seem  to  come  from  one  point  of  the  horizon.  The  radiant 
point  is  that  point  where  these  bands,  or  their  direction  pro- 
longed, meet  the  horizon.  The  position  of  this  point  on  the 
horizon  should  be  recorded  in  the  same  way  as  the  wind  direc- 
tion, N.,  NNE.,  and  so  on. 

"4.  Undulatory  clouds.  It  often  happens  'that  the  clouds  show 
regular  parallel  and  equidistant  striae,  like  the  waves  on  the 
surface  of  water.  This  is  the  case  for  the  greater  part  of  the 
cirro-cumulus,  str  at  o- cumulus  (roll-cumulus),  and  similar 
forms.  It  is  important  to  note  the  direction  of  these  striae. 
When  there  are  apparently  two  distinct  systems,  as  are  to  be 
seen  in  clouds  separated  into  balls  by  streaks  in  two  directions, 


118  THE  PRINCIPLES  OF  A&ROGRAPHY 

the  directions  of  the  two  systems  should  be  noted.  As  far  as 
possible,  observations  should  be  made  on  streaks  near  the  zenith 
to  avoid  effects  of  perspective. 

"5.  Density  and  position  of  cirrus  banks.  The  upper  clouds 
frequently  take  the  form  of  a  tangled  web,  or  of  a  more  or  less 
dense  veil,  which,  rising  above  the  horizon,  resembles  a  thin 
white  or  grayish  bank.  As  this  cloud  form  has  an  intimate 
relation  to  barometric  depressions,  it  is  important  to  note: 
"  (a)  The  density, — 

0  meaning  very  thin  and  irregular. 

1  meaning  thin  but  regular. 

2  meaning  rather  dense. 

3  meaning  dense. 

4  meaning  very  dense  and  of  dark  color. 

"  (b)  The  direction  in  which  the  veil  or  bank  appears  densest. 
"Remarks.     All  interesting  details  should  be  noted,  for  example: 
"  1.  On  summer  days  all  low  clouds  generally  assume  particular  forms 

more  or  less  resembling  cumulus.     In  this  case  there  should  be 

put  under  Remarks,  'Stratus  or  nimbus  cumuliformis.' 
"2.  It  sometimes  happens  that  a  cumulus  has  a  mammillated  lower 

surface.     This  appearance  should  be  described  by  the  name 

of  'mammato-cumulus.' 
"3.  It  should  always  be  noted  whether  the  clouds  appear  stationary, 

or  whether  they  have  a  very  great  velocity." 
Clayton,  in  the  Discussion  of  the  Cloud  Observations,  says 
that  "by  following  the  changes  in  nomenclature  since  Howard, 
it  seems  clear  that  there  has  been  a  gradual 
nomenclature  evolution,  during  which  differences  and  distinc- 
tions not  recognized  by  Howard  have  been  estab- 
lished, and  errors  due  to  perspective,  as  in  the  case  of  the 
cumulo-stratus,  have  been  corrected.  Thus  distinctions 
between  high  and  low  cirro-stratus  and  between  high  and  low 
cirro-cumulus  have  been  established,  and  the  lower  forms 
called  alto-stratus  and  alto-cumulus  respectively.  The  stratus 
has  been  separated  into  fog  and  low  sheet  clouds,  and  two 
distinct  forms  of  rain  cloud  are  recognized.  These  distinc- 
tions have  been  a  gradual  growth,  and  Abercromby  says: 
'At  Professor  Hildebrandsson's  suggestion  we  examined  the 
nomenclature  used  by  different  offices,  and  arranged  the  names 
systematically ;  and  we  found  that  the  differences  did  not  seem 
irreconcilable.  Eventually,  we  agreed  that  ten  terms,  all 
compounded  of  Howard's  four  fundamental  types, —  cirrus, 
stratus,  cumulus,  nimbus, —  would  fully  meet  the  requirements 


OF  ATMOSPHERE 


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THE  PRINCIPLES  OF  AEROGRAPHY 


WATER  VAPOR  OF  ATMOSPHERE  121 

of  practical  meteorology,  with  the  least  disturbance  of  existing 
systems.'1  Hildebrandsson  further  says  that  the  ten  cloud 
forms  described  were  already  recognized  in  the  nomenclature 
used  in  Portugal.  Hence  the  international  cloud  nomenclature 
adopted  at  Munich  represents  the  greatest  progress  in  cloud 
nomenclature  which  observers  are  yet  ready  to  accept  for 
general  use;  and  no  official  bureau  should  hesitate  to  accept  it 
for  fear  that  the  system  is  merely  temporary  and  will  soon  be 
changed.  Progressive  development  will  undoubtedly  continue, 
but  changes  of  names  in  general  use  will,  in  all  probability,  be 
slow.  A  more  detailed  nomenclature  is,  however,  needed 
for  the  use  of  specialists." 

36.  Distribution  of  the  various  types  of  clouds.  Bigelow 
shows  graphically  (diagrams,  Figs.  40  and  41)  the  distribu- 
tion of  the  several  types  of  clouds.  Under  the  name  of  each 
type  he  gives  the  mean  height  in  meters  for  the  year  and  the 
number  of  observations.  There  is  also  plotted  for  the  several 
types  the  curve  of  frequency,  with  heights  as  ordinates  and 
the  number  of  observations  at  the  respective  heights  as  ab- 
scissas. The  curves  follow  the  mean  line  of  the  plotted  points 
very  closely.  Under  the  assumption  that  the  observed 
frequency  corresponds  with  the  actual  frequency  of  cirrus 
formation  at  the  given  height,  a  discussion  of  these  curves 
would  give  a  good  explanation  of  the  physical  processes 
operative  in  cloud  formation  for  the  whole  year. 

In  the  diagrams  the  mean  heights  are  shown,  also  the  upper 
and  lower  limits.  There  is  a  wide  range  in  the  heights  of 
certain  clouds.  The  mean  height  of  the  low  clouds  is  probably 
2,000  meters.  The  three  low  cloud  strata  are  shallow  (not 
exceeding  3,000  meters  in  depth),  except  the  cumulo-nimbus, 
or  thunder  head,  which  may  develop  a  height  of  13,000  meters. 
All  the  clouds  except  stratus  and  cumulo-nimbus  show  a 
tendency  to  three  maxima  of  height  and  thickness,  one  in 
midsummer  and  the  other  two  in  February  and  November. 
The  minima  occur  in  March  and  September.  A  similar 
relation  is  found  to  exist  in  the  isothermal  limits,  as  pointed 
out  in  the  chapter  on  the  stratosphere. 

Cirrus  bands  have  been  explained  as  due  to  differences  in 

1  Quart.  Jour,  of  the  Royal  Met.  Soc.,  April,  1887,  p.  155. 


122 


THE  PRINCIPLES  OF  AEROGRAPH Y 


I 


WATER  VAPOR  OF  ATMOSPHERE  123 

velocity    or   in    direction   of    contiguous    upper-air    currents. 
These  currents  nearly  always  move  from  west  to  east,  and 
the  higher  part  of  the  current  may  move  more     _. 
rapidly   than   the   lower.     Thus   the   upper  part 
of  any  cloud  formation  might  move  in  advance  of  the  base, 
causing  a  band  or  bar  extending  from  west  to  east. 

37.  Wave  motions  in  the  air  shown  by  cloud  undulations. 
Cloud  billows,  or  undulations,  have  a  different  origin  from 

cirrus   bands,    though  it   is  not   always   easy   to     _, 
,.,..,,  A  /:,  Cloud  billows 

distinguish  between  the  two.  According  to  Clay- 
ton, bands  are  usually  isolated  or  widely  separated  and  are 
of  unequal  length,  while  undulations  are  close,  parallel  rows 
or  striations  of  nearly  equal  length.  The  undulations  were 
comparatively  little  observed  until  Helmholtz  called  attention 
to  them  as  illustrating  wave  motions  in  the  air,  of  the  same 
nature  as  ocean  waves.  These  undulations  are  visible  in  clouds 
at  all  altitudes.  They  are  illustrated  in  the  strato-cumulus 
by  long  parallel  rows,  which  are  parallel  in  fact  as  well  as  in 
appearance,  and  lie  in  approximately  the  same  direction  in  all 
parts  of  the  sky.  This  can  be  seen  by  laying  a  ruler  across  the 
center  of  the  mirror  of  the  nephoscope  parallel  with  the  undu- 
lations. Abercromby  had  a  different  opinion,  but  he  clearly 
made  no  critical  observations  in  this  way.  The  undulations 
are  illustrated  in  the  cumulus  level  by  long  parallel  lines 
formed  by  individual  cumuli  like  a  file  of  soldiers,  and  the 
lines  appear  to  converge  toward  the  horizon  as  the  effect  of 
perspective.  The  undulations  appear  to-  be  most  frequent 
in  the  alto-cumulus  level,  and  are  easily  distinguished  by  the 
parallel  rolls  in  the  alto-cumulus  and  by  striations  in  the  alto- 
stratus,  like  the  furrows  in  plowed  ground.  In  the  cirrus  level 
they  are  usually  distinguished  as  short,  parallel  threads  or  as 
small  bands  forming  one  broad  band  at  right  angles  to  their 
length.  Sometimes  they  seem  like  furrows  in  the  cirro-stratus. 
The  direction  of  length  of  the  undulations  is  decidedly  most 
frequent  from  north  to  south,  which  is  at  right 
angles  to  the  most  frequent  direction  of  cirrus  bands" 

bands.     It  leads  at  once   to   the  inference  that 
cloud  undulation  is  the  phenomenon  to  which  the  popular  name 
of  "polar  bands"  was  applied  in  Europe,  but  not  to  cirrus 


124 


THE  PRINCIPLES  OF  A&ROGRAPHY 


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MEAN  DIRECTION  OF  THUNDER-STORMS  IN  CYCI  ONES 


/6      T* 


Clayton  in  "Blue  Hill  Observations" 


"      KILOMETERS    8OO 

FIG.  42.     DISTRIBUTION  OF  CLOUDS  IN  CYCLONES  AND  ANTICYCLONES 

bands,  as  many  meteorologists  have  supposed  and  have  thus 
been  led  to  introduce  a  wrong  usage  of  the  term.  The  ten- 
dency for  the  undulations  to  lie  at  right  angles  to  their  motion 
is  very  distinct,  and  is  in  contrast  with  the  cirrus  bands. 


WATER  VAPOR  OF  ATMOSPHERE 


125 


/ 


I 


FIG.  43.     INSTRUMENT  FOR  MEASURING  CLOUD  HEIGHTS 

Since  the  crest  of  a  wave  usually  lies  at  right  angles  to  the 
wind  which  originates  and  drives  it  forward,  it  follows  that 
the  results  of  observation  agree  very  well  with       _ 
Helmholtz's  explanation  of  the  cloud  undulations;      crests  of 
namely,  that  the  clouds  are  the  visible  crests  of      atmospheric 
real  atmospheric  waves  formed  between  currents 
of  air  of  a  different  density  and  having  a  different  velocity  or 
direction.     The   undulations   would   probably   always   lie   at 
right  angles  to  the  upper  current  were  it  not  that  the  lower 
current  is  also  in  motion,  and  that  the  observed  cloud  direction 
is  a  compound  of  the  two. 

38.  The  value  of  clouds  in  forecasting  weather  changes. 
The  cloud  is  not,  as  might  be  expected  at  first  thought,  a 
good  exponent  of  air  motion;  and  as  yet  cloud  maps  have  not 
been   used   advantageously  by  professional  fore-     clouds 
casters  except  in  connection  with  storms  of  the     unreliable  in 
West  Indian  hurricane  type  or  the  typhoon,  or 
baguio,  of  the  China  Sea.     Thus  Father  Vines,  at  Havana, 
showed  how  certain  types  of  cirrus  accompanied  or  rather 
preceded  storms  of  great  violence  but  small  diameter,  while 
a   different   type  was  found  to   accompany   storms  of  large 


Clayton  and  Fergusson  at  Blue  Hi 

FIG.  44.     PLOTTING  MACHINE  FOR  MEASURING  CLOUD  HEIGHTS 


FIG.  45.     EDGE  OF  A  CUMULUS  CLOUD  McAdi 

126 


WATER  VAPOR  OF  ATMOSPHERE 


127 


diameter  and  moderate  violence.  Likewise  at  Manila, 
Zi-ka-wei,  and  other  observatories  on  the  Asiatic  coast,  the 
appearance  and  motion  of  the  upper  clouds  have  been  care- 
fully studied  for  forecasting  purposes. 

In  general,  cirrus  clouds,  except  of  a  certain  type,  do  not 
positively  indicate  coming  rain,  being,  in  fact,  somewhat  less 
frequently  followed  by  rain  than  the  average  probability  of 
rain.  But  they  are  closely  controlled  in  their  movements  by 


1 


FIG.  46.     CIRRUS 


Photograph  by  Ellerman,  at  Mt.  Wilson 


temperature    gradients,    and    they    may    serve    an    isolated 
observer  as  a  guide  to  coming  changes  of  temperature.     In 
general,    slowly    moving    cirrus    clouds    indicate 
slight  changes  in  temperature,  and,  except  when      clouds  and 

moving    from    a    direction    between    south    and      forecasts  of 
......  .  .  temperature 

west,    they    indicate,    as    a    rule,    slowly    rising 

temperature  during  the  succeeding  twelve  and  twenty-four 
hours.  Rapidly  moving  cirrus  indicate  the  probability  of 
decided  changes  of  temperature,  and,  from  any  direction,  a 


Photograph  by  Ellerman,  at  Mt.  Wilson 

CIRRUS  PLUMES 


FIG.  48.     CIRRUS 
128 


Photograph  by  Ellerman,  at  Sit.  Wilson 


WATER  VAPOR  OF  ATMOSPHERE 


129 


FIG.  49.     CIRRUS  BANDS 


Photograph  by  F.  R.  Ziel 


probability  of  a  fall  of  temperature  by  the  end  of  twenty-four 
hours.  The  probability  of  a  fall,  however,  and  the  amount 
of  fall,  are  much  greater  and  earlier  when  the  cirrus  are 
observed  to  be  moving  rapidly  from  a  direction  between 
south  and  west.  When  cirrus  are  observed  to  be  moving 
from  the  southwest,  there  is  a  strong  probability  of  a  fall  of 
temperature  during  the  succeeding  twenty-four  hours.  This 
probability  rises  to  83  per  cent  in  winter  and  is  over  70  per 
cent  at  all  times  of  the  year  for  cirrus  moving  rapidly  from 
the  southwest.  When  cirrus  are  observed  to  be  moving  from 
the  northwest,  the  probability  is  that  there  will  be  a  rise  of 
temperature  during  the  succeeding  twelve  hours.  The  proba- 
bility is  64  per  cent  for  winter  and  76  per  cent  during  the 
entire  year  for  cirrus  moving  rapidly  from  the  northwest. 

With  the  appearance  of  cirro-stratus  there  is  a  probability 
of  rain  during  the  succeeding  twenty-four  hours     cirro-stratus 
of  about  80  per  cent.     This  probability  increases     and  rain 
to  nearly  90  per  cent  with   the   appearance  of     Probablllty 
alto-stratus,  which  is  as  great  as  can  usually  be  derived  from 

10 


THE  PRINCIPLES  OF  A&ROGRAPHY 


FIG.  50.     CLOUD  FORMATIONS  IN  ADVANCE  OF  STORM  McAdie 

a  knowledge  of  the  conditions  prevailing  over  the  country 
as  given  on  a  weather  map.  Cirro-cumulus  are  most  fre- 
quently followed  by  fair  weather,  while  alto-cumulus  indicate 
a  probability  of  rain. 

There  are  two  directions  in  which  observations  of  the  direc- 
tion and  of  the  relative  velocity  of  upper  clouds  might  be  of 
When  cirrus  use-  The  rapid  movement  of  cirrus  from  the 
indicate  west,  or  the  southwest,  along  the  northern 

coming  cold  boundary  of  the  United  States,  will  no  doubt 
indicate  the  approach  of  a  cold  wave  before  its  approach  is 
indicated  by  the  weather  map,  and  will  thus  enable  north- 
western stations  to  be  warned.  The  movement  of  cirrus  from 
the  south,  observed  at  any  of  the  Atlantic  coast  stations 
with  a  dense  bank  of  clouds  to  the  south  of  the  observer, 
would  strongly  indicate  a  severe  storm  off  the  coast,  and 


WATER  VAPOR  OF  ATMOSPHERE 


131 


might  enable  the  observer  to  determine  the  position  of  its 
center.  This  conclusion  is  derived  from  the  individual 
observations  at  Blue  Hill,  and  from  the  fact  of  circulation 
of  air  in  cyclones  with  deep  barometric  depressions  (Fig. 
42).  The  prevalence  of  rapidly  moving  cirrus  over  a  wide 
area  indicates  rapid  storm  movement,  and  rapid  and 
marked  changes  of  weather  and  temperature.  Slowly  mov- 
ing cirrus  indicate  sluggish  storm  movements  and  slight 
changes  of  temperature,  and  are  the  usual  accompaniment 
of  droughts. 

The  direction  of  cirrus  movement  prevailing  in  advance  of 
and  around  the  storm  center  must,  in  the  majority  of  cases, 
furnish  a  clew  to  the  future  movements  of  the  storm,  since  it 
is  found  that  the  storm  tends  to  move  in  the  mean  direction 
of  the  cirrus  found  for  the  storm  as  a  whole. 

39.  Recording  sunshine.  For  recording  sunshine  during 
the  day  hours  various  instruments  are  used.  Some  are 


FIG.  51. 


Photograph  b 

CUMULUS  OVER  MOUNTAINS 


n,  at  Mt.  Wilson 


132 


THE  PRINCIPLES  OF  A&RQGRAPHY 


FIG.  52.     CAMPBELL-STOKES  SUNSHINE  RECORDER 


thermometric1  and 
because  of  their  re- 
sponsiveness to  heat, 
read  too  high  if  the 
temperature  contin- 
ues high  after  sunset, 
and  too  low  if  the 
temperature  falls 
rapidly  during  the 
day;  and  some  are 
photometric,  based 
chiefly  on  the  discol- 
oration of  sensitized 

paper.  At  Blue  Hill  there  has  been  in  constant  use  the  well- 
known  Campbell-Stokes  sunshine  recorder  (Fig.  52).  This 
record  consists  essentially  of  a  glass  sphere  which  focuses  the 
sun's  rays  and  burns  a  record  upon  a  strip  of  prepared  paper. 
This  instrument  is  also  used  to  obtain  a  record  of  moon- 
light by  placing  a  strip  of  photographic  paper,  not  over- 
sensitive, in  the  metallic  frame,  protecting  it  as  much  as 
possible  from  extraneous  light  and  from  the  weather.  Some 
very  good  records  have  been  thus  obtained,  and  it  is  quite 
easy  to  ascertain  by  these  means  the  intensity  of  the  illumina- 
tion due  to  the  moon.  An  interesting  record  (Fig.  53)  is 
that  of  March  11-12,  1914,  during  a  lunar  eclipse.  The  moon 
entered  the  penumbra  at  8:41  P.M.  and  entered  the  shadow 
at  9:42.  The  middle  of  the  eclipse  was  at  11:13  P.M.,  the 
moon  leaving  the  shadow  at  12:44  A.M.  and  the  penumbra  at 
1:45  A.M.  The  night  was  beautifully  clear.  On  the  record  it 
will  be  seen  that  the  light  gradually  became  dim,  disappearing 
about  11  o'clock  and  reappearing  after  midnight. 

A  record  (Fig.  54)  for  the  succeeding  night  shows  that 
there  was  moonlight,  with  intervals  of  cloudiness,  until 
10:50  P.M.,  after  which  the  moon  was  obscured.  In  the 

1  The  Maring  sunshine  recorder  used  at  Weather  Bureau  stations  is  essentially 
a  differential  air  thermometer.  Owing  to  greater  heat  absorption,  the  air  in  the 
black  bulb  expands  more  than  the  air  in  the  bright  bulb  and  the  thread  of 
mercury  is  forced  upward,  making  an  electrical  circuit,  when  it  meets  the  plat- 
inum tips.  A  circuit  breaker  in  the  clock  of  the  quadruple  register  records 
minutes  of  sunshine.  The  instrument  has  a  large  twilight  error,  and  must  be 
frequently  adjusted. 


WATER  VAPOR  OF  ATMOSPHERE  133 

illustration  the  time  scale  has  been  enlarged  to  twice  that  of 
the  other  record. 

The  whole  subject  of  cloudiness,   or  perhaps  it  would  be 
better  to  say  obscuration  by  clouds,   is  in  a  most  unsatis- 
factory state.     At  most  observatories  it  is  con-  Hours  for 
sidered  sufficient  to  enter  the  state  of  the  sky  recording  state 
twice  in  twenty -four  hours.     In  this  country  the  y 

hours  are  8  A.M.  and  8  P.M.,  but  such  a  practice  certainly  does 
not  afford  a  fair  basis  for  estimating  the  relative  cloudiness  of 
the  place.  On  the  other  hand,  a  systematic  survey  of  the 
cloud  forms  requires  endless  time  and  patience. 

One  looks  in  vain  through  textbooks  on  meteorology; 
official  circulars  of  instruction,  and  observers'  handbooks 
for  information  regarding  records  of  moonlight  and  night 
cloudiness.  In  fact,  cloudi-  .— __^^_____  „ 

ness  during  the  dark  hours,     7.00  P.M.     ,          .  ..  1.45  A.M. 

.      i,  ~  .  Lunar  Eclipse 

or  practically  half  the  time,  9.42P.M.  to  12.44.  A.M. 

is     discreetly     let     alone     by       FIG.  53.     MOONLIGHT  RECORD  DURING 

professional     meteorologists. 

If  recorded  at  all,  the  data  -  •»  »  *  .     — 

are    in     abbreviated     form,  7.00  P.M.  IO.SORM. 

and  are  generally  based  up-       FIG.  54.    MOONLIGHT  RECORD  WITH 

,  ,  CLOUD  INTERVALS 

on  hearsay,  such  as  state- 
ments of  night  watchmen  and  others.  Such  data  are,  of 
course,  of  doubtful  value.  And  yet  many  a  case  in  criminal 
and  civil  courts  requires  evidence  of  a  positive  character 
regarding  cloudiness  at  night,  and  particularly  the  illumination 
due  to  moonlight. 

For  getting  a  record  of  night  cloudiness,  there  has  been 
used  at  Blue  Hill  Observatory  for  many  years  a  pole- 
star  recorder.  This  is  a  photographic  record  of  Night- 
a  star  trail,  in  this  case  the  polestar,  devised  in  cloudiness 
1885  by  Professor  E.  C.  Pickering.  In  1904  the 
instrument  was  modified  by  Fergusson,  and  a  record  for  two 
weeks  may  be  obtained  on  one  film;  the  instrument  is  prac- 
tically automatic.  The  cost  is  not  large,  and  there  would 
seem  to  be  no  good  reason  why  the  instrument  should  not  be 
generally  used.  If  the  night  is  clear,  in  the  northern  portion 
of  the  sky,  the  trail  made  by  the  star  is  continuous.  For 


134 


THE  PRINCIPLES  OF  A&ROGRAPHY 


example,  in  Fig.  55  the  record  for  the  night  shows  that  the 
sky  was  entirely  clear  and  not  only  the  polestar  but  other 
star  trails  are  continuous.  Complete  cloudiness  would  be 
indicated  by  absence  of  trail.  In  latitude  45°  the  instrument 
gives  a  fair  record  of  night  cloudiness  for  the  entire  sky. 

While  it  is  important,  for  many  reasons,  that  cloudiness 
should  be  reported  with  some  detail,  still  the  records  should 
be  as  compact  as  possible.  A  good  illustration  of  a  handy 
form  of  record  is  shown  in  Fig.  56.  This  was  made  for 
a  lighting  company.  A  similar  record  for  the  succeeding 

month  showed  an 
entirely  different 
distribution  of 
cloudiness;  in  fact, 
the  month  was  so 
cloudy  that  the 
demand  for  light 
was  double  that 
of  the  preceding 
month.  At  Blue 
Hill  the  cloudiness 
for  each  hour 
throughout  the 
year  is  charted  on 
a  sheet  of  milli- 
meter paper  24 
centimeters  wide 
and  73  centimeters 
long.  The  record  of  cloudiness  at  Blue  Hill  is  probably  more 
complete  than  at  any  other  point  in  the  United  States. 

An  unusually  interesting  cloud  record  is  that  made  at 
Epsom,  Surrey,  by  S.  C.  Russell,  who  maintained  an  hourly 
record  for  eight  years.  During  this  period  he  accumulated 
nearly  a  hundred  thousand  individual  records.  He  has  pub- 
lished the  results  of  monthly  and  hourly  cloud-form  fre- 
quencies in  the  Quarterly  Journal  of  the  Royal  Meteorological 
Society,  October,  1913.  He  groups  the  clouds  into  four  main 
classes:  (1)  upper  clouds,  including  all  those  of  the  cirrus 
type'>  (2)  intermediate  clouds,  including  most  of  the  cumulus 


FIG.  55.     PICKERING'S  POLESTAR  RECORD,  BLUE 
HILL  OBSERVATORY 


WATER  VAPOR  OF  ATMOSPHERE 


135 


and  stratus  for- 
mations ;  (3)  lower 
clouds,  including 
fog;  and  (4)  diur- 
nal ascending  cur- 
rent formations. 
Curves  showing 
hourly  and 
monthly  frequen- 
cies are  given  at 
some  length.  Nor 
is  the  matter  of 
cloudless  periods 
omitted,  as  is  so 
often  the  case  in 
cloud  discussions. 
One  remarkable 
cloudless  period 
occurred  in  1909, 
when  from  6  P.M., 
April  4,  until  5 
P.M.,  April  11,  a 
period  of  167 
hours,  no  clouds 
were  visible.  The 
duration  of  the 
condition  of 
cloudlessness 
varies  greatly 
with  locality. 
On  the  Atlantic 
seaboard  periods 
exceeding  three 
days  are  rare ;  but 
in  the  far  western 
part  of  the  United 
States,  especially 
in  California  and 
Arizona,  periods 


136  THE  PRINCIPLES  OF  A&ROGRAPHY 

of  a  month  or  longer  without  cloudiness  are  not  infrequent. 
Langley  in  his  experiments  at  Mount  Whitney  speaks  of  the 
weeks  which  passed  without  a  cloud  in  the  sky;  and  it  is 
.  common  experience  in  the  high  Sierra  during 

cloudless          July,   August,    and    September  to  find   the  sky 

periods  in  entirely  cloudless,  week  after  week.  On  the 
the  Sierra  , J  .  '  .  11^- 

other  hand,  in  certain  seasons,  m  these  localities, 

thunderstorms  may  be  frequent  and  afternoon  cloudiness 
marked.  The  author  once  spent  a  week  on  the  summit  of 
Mount  Whitney  at  the  end  of  August,  and  more  than  half 
the  time  it  was  cloudy. 

Records  of  cloudiness  are  of  especial  interest  to  astronomers 
in  connection  with  total  eclipses  of  the  sun.  The  whole  pur- 
pose of  an  expedition  to  observe  an  eclipse  of  this  nature  may 
be  defeated  by  a  moment's  cloudiness.  When  the  path  of 
totality  is  such  that  a  choice  of  stations  can  be  made,  the 
astronomer  gives  preference  to  the  locality  having  a  record  of 
minimum  cloudiness. 


CHAPTER  XII 

CONDENSATION 

40.  The  formation  of  clouds  and  the  condensation  of 
aqueous  vapor.  We  have  seen  that  raising  a  cubic  meter 
of  unsaturated  air  a  little  over  100  meters  causes 
a  fall  in  temperature  of  one  degree.  This  has  of 
been  called  the  "adiabatic  rate,"  as  the  assump- 
tion is  made  that  no  heat  is  added  or  lost  through  other 
agencies.  We  have  also  seen  that  to  raise  the  temperature  of 
a  cubic  meter  of  dry  air  one  degree  requires  307  calories. 

When  a  mixture  of  air  and  vapor  is  lifted  there  will  be 
expansion  and  cooling  and,  because  of  the  cooling,  a  tendency 
toward  condensation  of  the  vapor.  Under  natural 
conditions  heat  may  be  added  or  lost  through 
convection,  radiation,  and  conduction.  Unlike 
a  solid,  a  gas  when  lifted  or  moved  from  a  place  of  high 
pressure  to  a  place  where  the  pressure  is  less  or  the  tem- 
perature higher,  expands;  work  is  done  and  heat  expended— 
not  in  the  lifting  of  the  air  or  gas,  but  in  overcoming  both 
internal  and  external  pressures.  Work  is  also  done  in  expan- 
sion when  the  mass  of  air  is  moved  horizontally.  In  a  perfectly 
homogeneous  atmosphere,  pressure  and  temperature  would 
both  decrease  with  height  and  offset  each  other  in  changing 
the  density  of  the  air.  We  should  then  have  established  a 
condition  of  unstable  equilibrium.  The  air,  if  given  a  dis- 
placement upward,  would  continue  to  move  upward,  remaining 
always  lighter  than  the  adjacent  air,  the  adiabatic  cooling 
not  being  enough  to  lower  the  temperature  to  that  of  the 
high  strata. 

Indifferent  or  adiabatic  equilibrium  occurs  when  the  cooling 

brings  the  temperature  to  that  of  the  new  level. 

T       ..  1  •  r  -11         •         /•••!••  When 

In   this   case  no  force  will   arise  facilitating  or       adiabatic 

opposing  the  displacement.     But  this  condition       equilibrium 

very  seldom  occurs  in  nature,  chiefly  because  of 

the  effect  of  water  vapor  and  the  temperature  change  due  to 

137 


138 


THE  PRINCIPLES  OF  A&ROGRAPHY 


condensation  or  evaporation.  As  soon  as  condensation  begins, 
the  heat  of  condensation  will  partly  offset  the  adiabatic  cooling, 
and  the  adiabatic  gradient  will  have  such  a  value  as  that 
given  in  the  following  table  from  Bjerknes,  which  is  a  modifi- 
cation of  the  one  given  by  Hann. 

ADIABATIC  FALL  OF  TEMPERATURE  PER  100  DYNAMIC  METERS  FOR 

SATURATED  AIR 


Pressure 
in  kbs. 

Temperature  degrees  absolute 

263° 

268° 

273° 

273° 

278° 

283° 

288° 

293° 

298° 

303° 

300 
400 
500 
600 
700 
800 
900 
1000 

0.52 
.58 
.63 
.67 
.70 
.72 
.75 
.77 

0.46 
.52 
.57 
.60 
.64 
.66 
.69 
.70 

0.40 
.45 
.49 
.54 
.57 
.59 
.62 
.64 

0.42 
.47 
.51 
.56 
.59 
.61 
.64 
.66 

0.42 
.46 
.50 
.54 
.56 
.59 
.61 

0.41 
.45 
.48 
.50 
.53 
.55 

0.37 
.40 
.42 
.45 
.48 
.50 

0.39 
.41 
.44 
.45 

0.38 
.40 
.41 

0.37 

.38 

adiabatic 
gradient 


While  the  adiabatic  gradient  for  dry  air  is  constant,  that  for 
saturated  air  varies  with  pressure  and  temperature,  decreasing 
Variation  of  with  pressure  fall  and  increasing  with  temper- 
ature fall.  As  Bjerknes  points  out,  the  decreases 
upward  both  of  pressure  and  of  temperature 
counteract  each  other  in  their  effect  on  the  fall  of  temperature, 
making  its  variation  with  height  gradual.  But  the  gradient 
will  increase  upward,  to  the  limit  1.0048,  which  would  be 
reached  when  all  moisture  had  fallen  out.  To  illustrate  this 
increasing  fall  of  temperature,  the  values  corresponding  to  the 
case  of  a  mass  of  air  with  initial  temperature  288° A.  near  sea 
level  and  moved  upward,  are  shown  by  the  underlined  values 
in  the  table.  It  will  also  be  noted  in  the  table  that  two 
values  are  given  for  the  freezing  point  as  the  transition  is 
made  from  the  solid  to  the  liquid  state.  Practically,  the 
value  0.5°  for  every  100  dynamic  meters  (101  meters)  is  used 
by  aerologists  as  the  average  value  of  the  temperature  gradi- 
ent for  saturated  air  in  the  lower  strata  of  the 
atmosphere.  If  temperature  gradients  are  less 
than  the  adiabatic,  we  have  a  state  of  stable 
equilibrium.  Under  such  conditions,  if  a  mass  of  air  be  carried 


Stable 
equilibrium 


CONDENSATION  139 

up  a  certain  distance,  the  adiabatic  cooling  will  bring  it  to  a 
lower  temperature  than  that  of  the  surrounding  masses  and 
it  will  fall  again  on  account  of  its  greater  density.  If  there 
should  be  no  gradient,  we  should  have  the  density  the 
same  throughout;  and  the  temperature  at  the  highest  level 
would  be  the  same  as  below.  This  would  be  known  as  an 
"iso  thermic  atmosphere."  Considering  extreme  cases  of  un- 
stable and  of  stable  equilibrium,  the  first  condition  would 
occur  if  the  fall  of  temperature  amounted  to  3.4°  for  every 
100  meters.  Such  gradients  may  exist  above  a  heated  sur- 
face and  probably  for  a  short  time  over  a  heated  area 

before  the  formation  of  a  tornado.     Contrasted 

.,-,,-,..,,  r  ,.  -.  Inversion 

with  this   is  the    case    of    a    negative   gradient, 

which  is  well  known  as  a  local  phenomenon,  under  the  name 
"  in  version."  It  is  a  condition  of  pronounced  stability  re- 
quiring some  effective  source  of  heat  to  overcome  it.  It  is 
important  in  connection  with  frosts  and  will  be  referred  to  later. 
Bjkernes  groups  the  various  states  in  four  classes : 

1.  The  homogeneous  atmosphere,  where,  with  a  pressure  of  100 

kilobars,  the  temperature  would  be  27°A.   and  the  gradient 
3.48°  per  100  dynamic  meters. 

2.  The  dry  atmosphere  in   adiabatic  equilibrium,   where,   with  a 

pressure  of  100  kilobars,  the  temperature  would  be  141°A.  and 
the  gradient  1°. 

3.  The  ordinary  saturated  atmosphere,  where,  with  a  pressure  of 

100  kilobars,  the  temperature  would  be  196° A.  and  the  gradient 
0.5°. 

4.  The  isothermic  atmosphere,  where,  with  any  pressure,  the  tempera- 

ture would  remain  the  same,  the  gradient  being  zero. 

These  may  also  be  arranged  according  to  dynamic  heights, 
in  which  case  we  should  have  at  an  elevation  of  1,000  dynamic 
meters  (1,000  ordinary  meters  are  980  dynamic  meters)  for 

1,  a  pressure  of  872  kilobars  and  temperature  238°A. ; 

2,  a  pressure  of  878  kilobars  and  temperature  263°A. ; 

3,  a  pressure  of  879  kilobars  and  temperature  268°A.; 

4,  a  pressure  of  880  kilobars  and  temperature  273°A. ; 

and  at  an  elevation  of  5,000  dynamic  meters,  for 

1,  a  pressure  of  362  kilobars  and  temperature  99°A.,  density  same 

as  at  sea  level ; 

2,  a  pressure  of  494  kilobars  and  temperature  223°A.,  density  60 

per  cent  of  that  at  sea  level; 


140  THE  PRINCIPLES  OF  A&ROGRAPHY 

3,  a  pressure  of  512  kilobars  and  temperature  248° A.,  density  56 

per  cent  of  that  at  sea  level ; 

4,  a  pressure  of  528  kilobars  and  temperature  273°A.,  density  50 

per  cent  of  that  at  sea  level. 

It  is  not  easy  to  apply  adiabatic  gradients  to  the  actual 
condition,  because  the  atmosphere  is  constantly  gaining  or 
losing  heat  through  horizontal  convection.  Bigelow  has 
deduced1  the  necessary  corrections  and  auxiliary  charts  where- 
by the  heat  divergence  between  the  assumed  and  the  actual 
can  be  quickly  obtained  for  purposes  of  forecasting.  He  also 
uses  the  temperature  gradient  in  connection  with  cumulus 
clouds.  The  adiabatic  gradient  .98°  A.  per  100  meters  is  not 
often  found  in  the  lower  strata.  The  bases  of  cumuli  usually 
form  higher  than  the  surface  values  would  indicate,  and  thus 
subheating  is  the  rule,  although  superheating  may  occur  on 
warm  days  near  the  ground.  Starting  at  the  surface  with  a 
given  dewpoint  and  following  on  a  Hertz  or  other  diagram 
the  line  of  constant  saturation  weight  (Fig.  57),  Bigelow 
deduces  certain  values  which,  with  selected  gradients,  will 
hold  throughout  the  given  height,  say  the  height  of  a  cumulo- 
nimbus or  thundercloud.  He  thus  employs  the  lofty  cloud  as 
a  gauge  for  obtaining  temperatures  at  high  levels. 

41.  Conditions  present  in  condensation.  Condensation  of 
the  vapor  into  water  decreases  the  rate  of  cooling  with  either 
Rate  of  cooling  elevation  or  lateral  convection  to  lower  pressure, 
decreased  by  If  the  condensation  is  carried  farther  and  snow 
condensation  results>  the  gradient  is  still  further  lessened.  In 
1884  a  brilliant  young  physicist,  Hertz,  expanded  the  small 
table  for  decrease  of  vapor  with  elevation,  as  given  by  Hann, 
and  introduced  a  graphic  method  of  following  the  changes 
in  moist  air.  He  discussed  four  stages :  first,  where  the  air  is 
Four  sta  es  unsaturated  and  no  liquid  water  is  present ;  second, 
accompanying  where  the  air  is  saturated  and  contains  also  addi- 
the  changes  tional  fluid  water;  third,  where,  in  addition  to 
in  moist  air  j  1  •  •  j  •  •  j  x  AI.  i. 

vapor  and  liquid,  ice  is  present ;  and  fourth,  where 

there  are  only  vapor  and  ice.  The  four  stages  are  designated 
also  as  the  dry,  the  rain,  the  hail,  and  the  snow  stage.  It 
will  frequently  happen  that  the  temperature  down  to  which 

1  Report  of  the  International  Cloud  Studies.  Also  Bigelow  Atmospheric  Circu- 
lation and  Radiation. ' 


CONDENSATION 


141 


the  first  stage  holds  good  will  lie  below  the  freezing  point; 
in  that  case,  one   passes  directly  over  to  the  fourth  stage. 


t4»*A       Z48  eS3  238 


Z93  KM  303*A 


£50  2*3  2*8  273  £78  £83  2S9  £93  £ 


1000 
kb 


FIG.  57.     ADIABATIC  DIAGRAM 

If  we  had  to  deal  with  only  one  mixture,  whose  composition 
was  exactly  known,  and  only  one  value  of  the  ratio  of  the 
weight  of  the  unsaturated  aqueous  vapor  to  the  weight  of  the 
dry  air,  then  we  could  represent  pressure  and  temperature  by 
coordinates  in  one  plane  and  cover  this  with  a  system  of 
curves  connecting  all  those  conditions  which  could  adiabatically 
occur.  But  the  aerographer  must  deal  with  mixtures  of 
varying  proportions.  We  can  get  along  with  one  graphic  table 


142  THE  PRINCIPLES  OF  A&ROGRAPHY 

if  we  confine  ourselves  to  those  cases  in  which  the  weight  and 
pressure  of  the  aqueous  vapor  are  small  in  comparison  with  the 
air.  One  cannot  expect  too  great  accuracy.  Then  the  same 

,       curve   can  be   used   for   different   mixtures;   but 
Methods  of  .  ...  . .  „ 

making  the  points  at  which  the  different  stages  pass  into 

dfa^ams  each  °ther  wil1  be  located  differently;    for  this, 

special  devices  are  needed.  The  solution  of  this 
problem  was  the  origin  of  the  Hertz  diagram.  Pressures  are 
laid  off  as  abscissas,  and  temperatures  as  ordinates.  The 
diagram  is  so  constructed  that  an  equal  increase  of  distance 
corresponds  to  an  equal  increase  in  the  logarithm  of  the 
pressure  and  in  the  logarithm  of  the  absolute  pressure. 

In  a  paper  presented  in  1900,  Neuhoff  modified  the  Hertz 
diagram  and  gave  both  numerical  and  graphic  methods  for 
adiabatic  changes  of  condition  of  moist  air.     A 
showing  new   diagram,    showing   the   diminution   of   tern- 

diminution  of  perature  with  altitude,  is  so  arranged  that  pressure 
wSfaltitude  *s  represented  by  cross  lines  with  a  slant  and  is 
read  by  scales  on  either  side  of  the  diagram.  The 
stages  are  the  same  as  those  in  the  Hertz  scheme.  In  the 
first  the  cooling  proceeds  without  saturation,  hence  without 
precipitation;  it  is  therefore  called  the  dry  stage.  In  the 
second,  saturation  occurs  accompanied  by  partial  conden- 
sation and  rain,  falling  or  suspended,  called  the  rain  stage. 
In  the  third,  the  temperature  has  fallen  to  273°A.  and  the 
precipitated  water  freezes,  while  at  the  same  time  partial  evapo- 
ration takes  place,  the  temperature  remaining  constant,  called 
the  hail  stage.  In  the  fourth,  all  the  water  is  frozen  and  the 
temperature  still  falls,  or  the  snow  stage  is  reached.  These 
processes  bring  about  different  final  results  according  as 
the  condensed  water  is  removed;  and  it  must  be  remembered 
that  in  a  reversed  process,  as  that  in  which  we  seek  to  follow 
the  changes  in  a  mass  of  descending  air,  owing  to  the  removal 
of  the  condensed  vapor  (in  the  form  of  rain,  snow,  or  hail),  the 
departures  from  initial  conditions  will  be  marked;  in  other 
words,  the  process  is  not,  strictly,  a  reversible  one.  Neuhoff 
starts,  not  with  the  usual  assumption  of  a  kilogram  of  moist 
air,  but  with  the  assumption  that,  the  condensed  aqueous 
vapor  being  constant,  then  the  weight  of  1  -j-  x  kilograms  of 


CONDENSATION  143 

moist  air  during  the  ascent  will  be  constant.  The  quantity 
x  is  the  vapor  that  is  mixed  with  the  kilogram  of  dry  air,  and 
is  designated  as  the  ''mixing  ratio,"  while  the 
quantity  of  aqueous  vapor  contained  in  1  kilogram  equations 
of  moist  air  is  the  specific  moisture.  For  high 
altitudes  the  mixing  ratio  is  very  small  (from  .001  to  .003) ; 
that  is  to  say,  there  are  from  1  to  3  grams  of  aqueous  vapor 
mixed  with  1  kilogram  of  dry  air.  Referring  to  the  adiabatic 
diagram  (Fig.  57),  dotted  lines  for  each  5  grams  indicate  the 
constant  quantity  of  moisture  needed  for  saturation.  The 
adiabats  of  the  dry  stage  are  the  straight  lines  running  parallel 
to  the  diagonals  of  the  small  squares,  while  those  of  the 
condensation  stage  are  the  curved  lines  indicated  by  dot  and 
dash.  An  example  of  the  use  of  the  diagram  is  as  follows: 
starting  with  a  pressure  of  1,012  kilobars,  temperature  293°  A. 
and  relative  humidity  80  per  cent  at  the  point  A,  the  amount 
of  vapor  needed  for  saturation  would  be  nearly  14  grams;  but 
as  the  relative  humidity  is  80  per  cent,  the  amount  present 
is  only  11.2  grams.  The  point  of  saturation  and  beginning 
of  condensation  will  be  found  by  following  the  adiabat  for 
the  dry  stage  until  it  intersects  the  gram  line  of  saturation 
representing  11.2  grams  at  the  point  B.  The  conditions  at  B 
at  the  end  of  the  dry  stage  are:  temperature  288° A.,  pressure 
of  vapor  at  saturation  17  kilobars;  amount  of  water  present 
11.2  grams,  pressure  of  atmosphere  960  kilobars;  and  altitude, 
500  meters.  The  air  is  now  saturated,  and  condensation 
begins.  If  the  temperature  falls  to  283° A.,  the  condensation 
adiabat  is  followed  from  the  point  B  to  its  intersection  with 
the  isothermal  of  283°  A.  at  C.  Here  the  conditions  are 
found  to  be:  pressure,  850  kilobars;  vapor  pressure  of 
saturation,  11  kilobars;  amount  of  water  vapor,  9  grams. 
Therefore  the  quantity  condensed  is  11.2—9=2.2  grams, 
the  altitude  1,500  meters.  When  the  temperature  falls  to 
273°  A.  the  rain  stage  ends.  This  is  shown  at  D;  and  the  corre- 
sponding pressure  is  650  kilobars;  vapor  pressure,  6  kilobars. 
The  quantity  of  vapor  is  6  grams,  showing  that  5  grams  have 
been  condensed  into  rain.  We  now  pass  to  an  altitude  of 
3,650  meters,  the  third  or  hail  stage,  where  there  is  a  constant 
temperature  until  all  the  water  is  frozen,  except  the  small 


144  THE  PRINCIPLES  OF  A&ROGRAPHY 

quantity  lost  by  evaporation.  To  freeze  the  6  grams  of  water 
still  left  requires  an  ascent  of  about  30  meters  per  gram;  and, 
therefore,  the  point  E  is  about  180  meters  higher,  or  about 
3,830  meters;  the  pressure,  630  kilobars;  the  vapor  pressure, 
6  kilobars;  the  amount  of  ice,  about  6  grams.  With  further 
cooling,  the  condensation  is  in  the  form  of  snow;  the  pressure, 
say,  500  kilobars,  or  half  the  original  pressure;  the  temperature, 
261° A. ;  the  weight  of  vapor  3  grams  and  of  frozen  water 
6  grams,  leaving  2.5  grams  as  snow  at  an  elevation  of  5,200 
meters.  The  diagram  is  faulty,  however,  in  that  the  rain 
and  snow  may,  and  generally  do,  fall  and  separate  from  the 
ascending  air. 

Air  that  is  saturated  at  an  initial  temperature  of  303°  A. 
at  a  pressure  of  1,000  kilobars  and  ascends  adiabatically  will 
have  a  temperature  of  freezing,  or  273° A.,  at  an  elevation  of 
7,360  meters.  If  the  initial  pressure  be  greater  the  height 
will  be  less,  approximately  100  meters  for  each  increase  of  30 
kilobars  initial  pressure. 

An  interesting  application  of  the  diagram  is  made  by 
Neuhoff  in  the  case  of  the  foehn  wind,  and  of  course  might 
also  be  made  in  connection  with  the  other  winds  of  this  type, 
such  as  the  chinook  and  the  well-known  Santa  Ana,  or 
"norther,"  of  southern  California.  While  in  the  case  of 
ascending  air  the  condensed  vapor  may  or  may  not  separate 
from  the  air  column  and  move  to  another  locality,  in  the 
case  of  descending  air  the  presence  or  absence  of  the  vapor 
is  of  the  utmost  importance  in  connection  with  the  heat 
developed  by  compression.  If  the  air  has  originally  passed 
over  a  mountain  ridge  and  lost  much  of  its  vapor  load  in  the 
shape  of  rain  or  fog  or  low  cloud,  or  even  snow,  then  we  can 
no  longer  pass  backward  over  the  adiabats.  During  the 
descent  of  the  air  no  vapor  is  removed  unless  by  wind  action, 
and  we  can  use  only  the  dry-stage  adiabat.  Neuhoff  gives 
the  following  example:  At  an  initial  temperature  of  287° A. 
and  a  relative  humidity  of  60  per  cent  we  find  the  saturation 
curve  for  10  grams,  and  hence 

10  X  60  +  100  =  6  grams  for  the  mixing  ratio. 

If,  now,   the  air  expands  adiabatically,   then,   as  shown  by 
the  intersection  of  the  diagonal  for  the  dry  adiabat  with 


CONDENSATION 


145 


the  6-gram  saturation  line,  the  air  will  have  a  temperature 
of  278°  A.  at  900  meters'  elevation.  If  further  expansion 
takes  place,  the  air  follows  the  condensation  adiabat  and 
has  a  temperature  of  273°  A.  at  1,750  meters,  and  the  satu- 
ration value  is  then  4.6  grams.  If  the  air  rises  to  a  mountain 
ridge  of  3,000  meters,  the  temperature  is  265° A.  and  the 
saturation  quantity  3  grams.  Now,  at  a  point  slightly  over 
the  summit,  the  precipitation  is  heaviest.  Let  us  assume 
that  the  total  quantity  of  moisture  remaining  is  3  grams. 


METERS 

4000 


3000 


1000 


1OOO 


10OO 


1000 


763°A     173°       385        293 

FIG.  58.     FOEHN  WIND 


303 


780°A       79O°A      300  °A 

FIG.  59.     FOEHN  ADIABAT 


FIG.  60.     MOUNTAIN  WINDS  DRIED  AND  WARMED 
IN  DESCENDING 

As  the  air  descends  on  the  other  flank  of  the  mountain  there 
is  adiabatic  compression;  and  if  we  follow  the  dry  adiabat, 
the  temperature  at  the  end  —  that  is,  at  the  initial  height  - 
would  be  295°  A.  At  this  point  it  would  require  17  grams  to 
produce  saturation;  therefore  the  relative  humidity  is  only 
18  per  cent,  and  the  air  is  both  warmer  and  drier.  The 
above  diagrams  (Figs.  58-60),  based  upon  diagrams  by 
Neuhoff  and  Wegener,  may  be  interesting. 

For  further  remarks  on  the  foehn,   chinook,   Santa  Ana, 
and  also  the  sirocco  and  hot  "northers"  of  the  western  plains, 

11 


146  THE  PRINCIPLES  OF  A&ROGRAPHY 

see  the  chapter  on  winds,  where  also  are  discussed  the  cold 
winds,  the  mistral  of  southern  France,  the  bora  of  the 
Adriatic,  the  pampero  of  Argentina,  and  the  southerly  burster 
of  Australia. 

In  several  memoirs  on  the  thermodynamics  of  the  atmos- 
phere, von  Bezold  treats  of  the  changes  which  a  given  mass 
of  air  undergoes  with  elevation  or  horizontal  con- 
vection from  higher  to  lower  pressure  provided 
it  is  not  affected  by  mixing  with  another  mass  of  different 
temperature  and  moisture  content.  But  mixing  is  constantly 
in  operation,  and  it  is  therefore  necessary  to  modify  materially 
the  equations  in  question. 

James  Hutton,  of  Edinburgh,  was  apparently  the  first  to 
give  attention  to  the  problem  of  mixing  in  connection  with 
the  formation  of  rain.  He  was  also  the  first  to  use  the  wet- 
bulb  thermometer  (used  later  by  Leslie  independently),  and 
may  have  been  led  to  his  conclusions  on  the  origin  of  rain 
from  his  studies  of  temperatures  of  evaporation.  Hutton 
held  that  the  mixing  of  two  masses  of  air  near  saturation  but 
of  different  temperature  resulted  in  precipitation.  For  many 
years  this  view  was  accepted;  then  it  was  questioned,  its 
opponents  going  so  far  as  to  say  that  no  precipitation  what- 
ever could  thus  occur.  As  usual  in  such  cases,  the  truth  lies 
between  the  two  extremes  of  opinion.  Hann,  in  1874,  proved 
that,  by  mixture,  condensation  could  indeed  be  produced, 
but  that  the  method  originally  used  for  computing  the  amount 
of  rainfall  was  inaccurate  and  that  when  proper  corrections 
are  applied  the  amount  of  precipitation  is  greatly  dimin- 
ished. In  fact,  the  precipitation  produced  in  this  way  can 
never  be  very  great.  Pernter,  in  1882,  computed  various 
tables  giving  the  possible  quantities,  and  later  von  Bezold, 
reviewing  the  whole  subject  of  the  formation  of  precipitation, 
discussed  the  various  causes,  such  as  direct  cooling,  adiabatic 
expansion,  and  mixture.  He  showed  that  a  mixture  of 
saturated  warmer  air  with  unsaturated  cooler  air  gives  more 
condensation  than  a  mixture  of  saturated  cooler  air  with  drier, 
warmer  air.  The  flowing  of  a  stream  of  saturated  warm  air 
into  cool  space  is  accompanied  with  more  condensation  than 
the  reverse  process.  This  explains  why  clouds  of  vapor 


CONDENSATION  147 

form  so  readily  over  an  open  warm-water  surface  while  the 
formation  of  fog  over  cold  surfaces  is  not  so  marked.  Von 
Bezold  illustrates  the  difference  by  calling  attention  to  the 
fact  that,  although  the  opening  of  a  wash-house  door  during 
moderately  cool  weather  causes  great  clouds  of  vapor  to 
pour  out,  the  opening  of  an  ice  cellar  on  a  warm  day  has  not 
a  similar  result. 

To   determine   the   maximum   precipitation   possible   from 
mixing    at    different    temperatures,    von    Bezold    conditions 
has  constructed  a  set  of  twelve  tables,  of  which    for  maximum 
an    abstract   follows,     somewhat   modified   from    PreciPltatl° 
the  original: 

Pressure  933  kilobars,  difference  of  temperatures  in  20-degree  units: 

First  mass  at  temperature  273°A.,  saturation  100  per  cent;  second 

mass  at  temperature  253°A.,  saturation  100  per  cent;  maximum 

possible  precipitation,  0.4  gram;  final  temperature  of  mixture, 

264°A. 
First  mass  at  temperature  283°A.;  second  mass  at  263°A.,  both  at 

saturation;  maximum  precipitation,  0.5  gram;  final  temperature, 

274°A. 
First  mass  at  temperature  293°A.;  second  mass  at  273°A.,  both 

at  saturation;  maximum  amount  precipitation,  0.7  gram;  final 

temperature,  284°A. 

Pressure  933  kilobars,  difference  of  temperatures  in  10-degree  units: 

First  mass  at  temperature  263°A.,  second  mass  at  temperature  253°A., 

both   at   saturation;   maximum   precipitation,    .04   gram;   final 

temperature,  257°A. 
First  mass  at  temperature  273°A. ;  second  mass  at  temperature  263°A. ; 

saturation,  100  per  cent;  maximum  precipitation,  .1  gram;  final 

temperature,  269°A. 
First  mass  at  temperature  283° A. ;  second  mass  at  temperature  273°A. ; 

saturation,  100  per  cent;  maximum  precipitation,  .2  gram;  final 

temperature,  278° A. 
First  mass  at  temperature  293°A. ;  second  mass  at  temperature  283°A. ; 

saturation,  100  per  cent;  maximum  precipitation,  .2  gram;  final 

temperature,  287°A. 

These  tables  show  how  small  is  the  precipitation  obtained 
by  mixture.     A  slight  direct  cooling  will  produce,    cooling  more 
therefore,  as  much  precipitation  as  a  considerable    effective  than 
cooling  due  to  mixture  with  cooler  air  even  when 
saturated.     The  following  is  an  illustration :     If  the  first  mass 


148  THE  PRINCIPLES  OF  ARROGRAPHY 

has  a  temperature  of  293° A.,  and  the  second  of  273° A.  (third 
case  above),  the  maximum  precipitation  would  be  .7  gram 
and  the  final  temperature  284° A.,  indicating  a  cooling  of  the 
whole  volume  9°.  By  direct  cooling  of  a  single  mass,  the 
same  amount  of  precipitation  could  be  obtained  by  a  fall 
from  293°  to  292°;  and  by  adiabatic  expansion, —  that  is, 
cooling  due  to  ascension,  diminished  pressure,  and  expansion 
at  a  temperature  of  291°,  which  would  occur  when  the  mass 
was  lifted  about  300  meters.  This  example  shows  how  slight 
need  be  the  cooling  (direct)  by  contact  with  cold  surfaces,  or 
through  radiation,  or  even  by  rising,  to  produce  condensation 
and  precipitation  equivalent  to  extensive  mixing. 

Aqueous  vapor  can  exist  in  a  supersaturated  state,  a  con- 
dition most  likely  to  occur  when  the  air  is  unusually  pure 
and  free  from  dust  or  nuclei  on  which  condensa- 
saturation  tion  might  occur.  This  will  be  referred  to  later 
in  connection  with  the  experiments  of  Aitken, 
Kiessling,  Robert  von  Helmholtz,  Wilson,  Simpson,  and 
others.  Condensation  and  precipitation  occur  in  such  super- 
saturated masses  of  air  when  nuclei  are  present  or  when 
electric  discharges  take  place. 

42.  Formation  of  fog  at  sea.  In  1913  a  special  investiga- 
tion of  ice  conditions,  meteorology,  and  oceanography  was 
undertaken  by  the  Board  of  Trade  of  the  British  govern- 
ment for  that  part  of  the  North  Atlantic  traversed  by  the 
large  liners.  The  steamship  Scotia,  a  whaler  rigged  as  a 
three-masted  bark  with  auxiliary  steam,  spent  about  fifteen 
weeks  off  the  coasts  of  Newfoundland  and  Labrador.  From 
the  report  of  the  meteorologist,  Mr.  G.  I.  Taylor,  the  follow- 
ing notes  are  taken  regarding  the  temperatures  and  humidities 
Balloon  recorded  during  the  formation  and  dissipation  of 

record  over  fog.1  A  captive-balloon  ascent  made  on  Aug- 
the  Atlantic  ugt  4  1913  ig  particuiariy  interesting;  for  it  is 

possible  to  trace  therefrom  the  effect  on  the  temperature- 
height  and  humidity  curves  of  two  different  changes, —  one 
from  cooling  to  warming,  and  the  other  from  warming  to 
cooling  again.  In  Fig.  61  is  shown  the  vessel's  course  and 

1  This  work  has  now  been  taken  over  by  the  U.    S.  Coast   Guard   vessels 
Seneca  and  Tampa  (formerly  the  Miami). 


CONDENSATION 


149 


position  on  preceding  days.  The  center  of  a  depression  had 
passed  over  Belle  Isle  a  few  days  before,  the  wind  changing 
from  one  direction  to  another.  The  temperature  changes 
were  probably  as  follows:  On  July  29  the  air  blew  from  the 
land  and  was  cooled  by  the  water  near  the  coast.  Then  it 
blew  over  warmer  water  until  the  morning  of  August  3,  when 
the  wind  changed  back  toward  the  cold  water.  The  effect 
of  these  changes  is  shown  in  Fig.  62.  From  sea  level  to  370 
meters  there  is  a  negative  temperature  gradient  (an  increase 
in  temperature  with  height,  generally  called  an  inversion) 


FIG.  61.     SEA  TEMPERATURE  AND  PATH  OF  AIR  DURING  FOG  OFF  NEWFOUNDLAND, 
S.  S.  SCOTIA,  AUGUST  4,  1913 

corresponding  with  the  cooling  from  8  A.M.,  August  3,  until 
the  time  of  the  ascent.  From  370  meters  to  770  meters  the 
temperature  gradient  is  positive  and  corresponds  with  the 
warming  which  the  air  experienced  from  July  30  to  August  3. 
Above  770  meters  the  gradient  is  negative  again,  and  cor- 
responds with  the  cooling  as  the  air  blew  off  the  land  on 
to  the  Arctic  current. 

The  humidity  curve  indicates  that  the  high  temperatures 
in  the  highest  layers  explored  were  due  to  hot  land  rather 
than  hot  sea;  the  instrument  registered  the  extremely  low 
humidity  of  20  per  cent  at  the  greatest  height. 

Measurements  were  made  three  times  daily  during  May, 


150 


THE  PRINCIPLES  OF  A&ROGRAPHY 


1915,  aboard  the  ice-patrol  cutter  Seneca,  of  the  number  of 
persistent  nuclei  in  the  air  per  cubic  centimeter  by  the 
corona  method  of  Barus.  Wells  states  that  the  number  was 


After  Taylor 

FIG.  62.     SEA  FOG  TEMPERATURE.     CONDITIONS  DURING  FOG,  AUGUST  4,  1913, 

7  P.  M.,  S.  S.  SCOTIA 

Weather,  thick  fog;  wind  direction,  S.  E.  K  S.;  wind  velocity,  five  miles  per  hour 
at  all  heights.  The  position  of  the  arrow  (  )  in  the  temperature-height  diagram 
represents  the  temperature  of  the  sea. 

found  to  be  never  less  than  400,  normally  1,000,  and  on  three 
occasions  as  high  as  50,000.  The  nucleation  was  generally  high 
in  cyclonic  areas,  leading  to  the  inference  that  the  nuclei  at 
sea  are  chiefly  salt  particles;  i.  e.,  evaporated 
spray.  The  amount  of  water  in  a  cubic  meter 
of  fog  was  found,  by  evaporating  the  fog  elec- 
trically and  measuring  the  humidity  at  the  higher  temper- 
ature, to  be  0.7  gram.  The  fog  particles  were  found  to 
have  a  diameter  of  the  order  of  .0005  cm.  A  rise  of  1.4°  C. 
in  temperature  would  dispel  this  fog;  and  therefore  a  slight 
temperature  "inversion"  resulted  in  a  shallow  fog,  not  ex- 
tending as  high  as  the  masthead. 

Von   Bezold   considers   the   following   fogs   and   clouds   as 
originating  by  mixture: 


Marine 
fog 


CONDENSATION  151 

"The  fog  above  warm,  moist  surfaces,  under  the  influence  of  colder 
air,  such  as  winter  fogs  at  sea. 

"The  'rank  and  file  clouds/  occurring  on  the  boundary  of  two  strata 
flowing  rapidly  above  each  other;  and  which  Von  Helmholtz 
first  recognized  as  a  consequence  of  air  motion  and  designated 
by  the  name  'atmospheric  billows,'  in,  which,  however,  adiabatic 
condensation  also  occurs  at  places  where  the  air  is  thrown  upward 
after  the  manner  of  the  formation  of  crests  and  foam  on  ocean 
waves. 

"The  layers  of  stratus  that  also  form  at  such  separating  surfaces. 
Cloud  streamers  that  form  and  again  dissolve  at  the  summits  of 
mountains  or  in  narrow  mountain  passes  when  the  topography 
is  such  as  permits  interpenetration  of  warm  and  cold  air  masses. 

"The  ragged  clouds,  or  the  disconnected  clouds,  such  as  occur  during 
rapid  motions  of  the  air,  perpetually  changing  their  forms  and 
appearing  and  disappearing." 

It  is  because  of  these  processes, — condensation,  evaporation, 
compression,  and  expansion —  that  the  motion  of  a  cloud 
does  not  necessarily  give  a  true  measure  of  the  motion  of 
the  air;  for  sometimes  clouds  hang  apparently  motionless 
on  the  mountains  while  strong  winds  stream  ,througl*Cthem 
(for  example,  the  foehn  cloud  bank,  the  "  Tablecloth "  of 
Table  Mountain,  the  cloud  cap  of  the  Helm  wind,  and 
others) ;  and  again,  balloonists  moving  horizontally  pass 
through  long  stretches  of  cloud  masses.  Within  the  cloud 
masses  themselves  marked  circulation  may  occur.  Obser- 
vations by  balloonists  passing  through  large  cumuli  show 
that  there  are  movements  within  the  cloud  that  are 
independent  of  the  general  drift  of  the  cloud  as  a  whole,  and 
that  small  bodies  are  whirled  up  and  down,  sometimes  vio- 
lently. In  cumulo-nimbus,  or  thunder  clouds, 
there  is  much  turbulence  irrespective  of  the  pro-  within  cloud 
gressive  motion  of  the  storm.  It  is  difficult, 
however,  to  obtain  reliable  and  sufficiently  detailed  records 
of  air  motion  as  well  as  the  physical  changes  in  temperature, 
humidity,  and  pressure.  It  may  be  pointed  out  that  in  so 
familiar  a  problem  as  recording  temperature,  our  thermom- 
eters tell  only  a  part  of  the  story  of  the  heat  change;  and 
our  instruments  for  recording  humidity  are  even  less  satis- 
factory than  the  thermometers.  The  ordinary  mercurial 
thermometer  indicates  simply  the  difference  in  expansion 


152  THE  PRINCIPLES  OF  AEROGRAPHY 

of  a  small  quantity  of  mercury  at  the  bulb,  and  of  the 
glass.  It  may  not  even  correctly  give  the  temperature  of 
the  surrounding  air;  and  of  course  it  tells  us  nothing  of 
the  heat  energy  gained  or  lost  in  any  change  of  state  of  the 
water  vapor  when  air  and  vapor  mix.  The  so-called  "latent" 
heat  (the  name  came  into  use  at  a  time  when  heat  was  thought 
to  be  a  material  flow  and  is  therefore  misleading),  the  heat 
accompanying  change  from  vapor  to  liquid  and  from  liquid 
to  solid,  a  change  that  is  of  constant  occurrence  in  nature,  is 
not  indicated  by  any  present  form  of  thermometer.  It  may 
be  well  to  repeat  here  that  latent -heat  energy  may  not  always 
appear  or  reappear  in  the  form  of  heat.  Thus,  as  we  shall 
see  in  the  frost  problem,  it  would  be  a  mistake  to  regard  the 
heat  of  condensation  as  actually  set  free  and  causative  of  a 
rise  in  temperature.  It  is  the  kind  that  may  be  utilized  in 
work.  The  energy  thus  freed  probably  goes  to  reinforce  the 
molecular  energy  expended  by  the  body  in  cooling.  It  thus 
tends  to  retard  the  rate  of  cooling.  It  does  act  to  prevent 
lowering  of  the  temperature,  but  not  as  a  quantity  of  heat 
directly  available  in  raising  the  temperature. 

43.  The  dissipation  of  aqueous  vapor.  We  pass  now  from 
the  consideration  of  problems  dealing  with  the  formation 

of  fog,   cloud,   and  rain  to  the  reverse  problem 
Evaporation  fe'.  .  f.   . 

of   dissipation  of  the  vapor  in  either  visible  or 

invisible  form.  This  brings  us  to  the  process  of  evaporation 
as  contrasted  with  the  reverse  process  of  condensation. 

If  a  mass  of  unsaturated  air  is  supplied  with  water,  there 
will  be  cooling,  and  the  degree  of  cooling  will  vary  with  the 
dryness,  being  greater  as  the  saturation  deficit  is  greater;  or, 
in  other  words,  the  greater  the  amount  of  water  evaporated, 
the  greater  the  degree  of  cooling. 

Von  Bezold  states  from  experience  that  frequently  when 
passing  through  strata  of  fog  such  as  fill  the  mountain  valleys 
on  calm,  clear  mornings,  as  one  ascends  the  valley,  the 
Low  tempera-  impression  of  colder  weather  occurs  immediately 
ture  before  on  reaching  the  upper  limit  of  the  fog.  It 
dissipation  frequently  happens  that  just  before  the  sun  dis- 
sipates the  morning  fog,  a  sensation  of  cold  is  experienced. 
He  explains  these  phenomena  on  the  assumption  that  the 


CONDENSATION  153 

temperature  just  below  the  upper  boundary,  when  the  fog 
is  dissolving,  is  lower  than  that  above  or  below.  For  when 
the  sun  begins  to  warm  the  upper  side  of  the  fog  there 
occurs,  first,  relative  dryness,  which  will,  according  to  the 
rapidity  of  evaporation,  extend  somewhat  into  the  fog  layer. 
In  other  words,  the  layer  of  fog  just  under  the  upper  surface 
has  the  lowest  temperature;  and  above  this  the  tempera- 
ture rises  and  the  humidity  falls.  This  relation  was  strikingly 
brought  out  by  Sigsfield,  in  a  balloon  trip  over  southern 
Germany,  October  26,  1889.  Gross,  who  also  made  a  balloon 
voyage  in  the  same  year,  found  that  in  passing  through  thick 
clouds  the  temperature  fell  very  low  at  the  upper  part  of  the 
cloud,  but  above  this,  rose  decidedly. 

In  the  cloud  work  at  Blue  Hill  it  was  frequently  noticed 
that  the  tops  of  cumuli  were  colder  than  was  the  air  at  a 
corresponding  level  at  the  same  time. 

In  the  matter  of  formation  and  dissolution  a  fog  is  but  a 
cloud  resting  on  earth,  while  a  cloud  is  simply  a  raised  fog. 
A  steady  increase  in  cooling  or  warming,  if  by    Evaporation 
radiation  or  expansion  or  compression,  is  accom-    increase  with 
panied  by  a  steadily  increasing  evaporation  or    warmms 
condensation;  but  in  the  case  of  mixture  the  process   can 
continue  and  yet  cause,  first  condensation,  and  later  evapora- 
tion.    The  breath,  exhaled  into  cool  air,  leaves  the  mouth 
saturated  but  not  condensed;  but  on  mixing  with  the  air  it 
is   chilled,    and   we   see   the   vapor    in    visible    form.     With 
further  mixture  with  cool,   dry  air  it  dissolves.    Mixtures 
If  we  mix  saturated  cooler  air  with   increasing    that  cause 
amounts  of  warmer  air,  then  the  warming  of  the    dissolution 
mixture  proceeds  more  rapidly   at    first    than  subsequently, 
whereas  in  the  reverse  process  cooling  proceeds  more  slowly 
at  first,  and  faster  afterwards. 


CHAPTER  XIII 
DUST  AND  MICROBES 

44.  Foreign  matter  in  the  atmosphere.  Thus  far  we 
have  dealt  with  the  atmosphere  as  though  it  were  made  up 
only  of  gases  and  water  vapor.  It  consists  of  something 
more  than  these.  There  is  in  suspension  in  the  atmosphere 
a  large  amount  of  organic  and  inorganic  matter.  Moreover, 
these  affect  the  processes  of  condensation  and  evaporation. 
They  have  not  been  studied  much  in  connection  with  the 
latter  physical  process,  but  in  condensation  their  influence  is 
marked  and  has  been  exhaustively  studied  by  Aitken,  Barus, 
and  others.  The  necessity  of  the  presence  of  nuclei  before 
condensation  can  occur,  as  pointed  out  by  Aitken,  will  be 
referred  to  in  a  forthcoming  paragraph. 

The  foreign  matter,  sometimes  called  the  impurities  of  the 
air,  is  composed  chiefly  of  microorganisms  and  dust.  Bacteria 
Bacteria  abound  in  large  numbers  in  the  atmosphere  of  a 

in  the  city,  and  much  work  has  been  done  by  medical 

atmosphere  investigators  to  trace  relationship  between  vitiated 
air  and  disease.  For  many  years  it  was  held  that  there  was 
a  direct  relation  between  the  two;  but  the  later  studies  and 
experiments  do  not  entirely  confirm  this  view.  We  may  not 
go  into  the  discussion  at  length,  but  it  may  be  of  interest  to 
quote  here  the  conclusions  of  Hill  and  others,  based  on  experi- 
ments at  the  physical  laboratory  of  the  London  Hospital 
Medical  College,1  on  the  influence  on  health  of  the  atmosphere 
in  confined  and  crowded  places.  In  brief  these  are: 

"No  symptoms  of  discomfort,  fatigue,  or  illness  result  from 
air  rendered  in  the  chemical  sense  impure  by  the  presence  of 
human  beings,  so  long  as  the  temperature  and  moisture 
are  kept  low.  Such  air  can  be  borne  for  hours  without  any 

decent  experiments  by  Eastman  and  Lee  referred  to  in  Science,  Aug.  11, 
1916,  p.  183,  show  that  the  harmfulness  of  respired  air  is  not  due  to  its  chemical 
components.  See  also  the  researches  of  the  New  York  State  Commission  on 
Ventilation,  published  in  American  Journal  of  Public  Health,  Vol.  V  (1915),  p.  85. 
Little  can  be  said  at  present  regarding  the  effect  of  atmospheric  conditions  on 

154 


DUST  AND  MICROBES  155 

evidence  of  bodily  or  mental  depression.  .  .  .  Heat 
stagnation  is,  therefore,  the  one  and  only  cause  of  the  dis- 
comfort, and  all  the  symptoms  arising  in  the  so-called  vitiated 
atmosphere  of  crowded  rooms  are  dependent  on 
heat  stagnation.  The  moisture,  stillness,  and  and^disease 
warmth  of  the  air  are  responsible  for  all  effects; 
and  all  the  efforts  of  the  ventilating  engineer  should  be  directed 
toward  cooling  the  air  in  crowded  places  and  cooling  the  bodies 
of  the  people  by  setting  the  air  in  motion.  The  essentials 
required  of  any  good  system  of  ventilation  are  (1)  movement, 
coolness,  proper  degree  of  relative  moisture  of  the  air,  and 
(2)  reduction  of  the  mass  influence  of  pathogenic  bacteria. 
The  chemical  purity  of  the  air  is  of  very  minor  importance." 

Air  laden  with  bacteria  is  generally  considered  offensive  and 
conducive  to  the  spread  of  disease.  In  certain  hospital  wards 
measurements  of  bacteria  present  have  shown  that  fifty  thou- 
sand recognizable  organisms  may  exist  in  a  cubic  meter  of  air. 

The  principal  sources  of  atmospheric  dust  are  volcanic 
action,  attrition  of  wind  on  land  and  sea,  evaporation,  and 
combustion.  The  movement  of  soil  material  Dust  content 
by  the  winds  is  much  greater  than  is  generally  of  the 
supposed,  and  even  in  open  places  dust  is  being  a  m 
deposited  and  removed  at  a  more  rapid  rate  than  in  places 
where  its  presence  is  immediately  perceptible.  The  move- 
ment of  soil  material  by  the  wind  is  described  in  great  detail 
by  Free  and  Stuntz,  in  Bulletin  No.  68  of  the  Department  of 
Agriculture,  Bureau  of  Soils.  Students  of  geology  know  of 
the  extensive  influence  of  dust  in  such  problems  as  dune 
control,  desert  geology,  the  occurrence  of  the  loess,  and 
volcanic  dust.  Black,  at  Edinburgh,  in  1902  showed  that 
the  amount  of  dust  deposited  in  an  open  rain  gauge  having 
a  funnel  150  millimeters  in  diameter  varied  from  1.62  grams 
to  10.37  grams  per  month.  The  average  was  2.7  grams,  which 
is  equivalent  to  150  grams  per  square  meter  per  month,  or 

the  nervous  system,  nor  do  we  know  much  about  the  relation  between  metabolic 
phenomena  and  atmospheric  conditions. 

Huntington  in  his  book  on  Civilization  and  Climate  has  shown  that  the  max- 
imum physical  efficiency  occurs  at  intermediate  seasons,  and  that  the  optimum 
temperature  of  the  outside  air  for  the  physical  work  of  human  beings  is  288°  A. 
and  for  mental  work  277° A. 


156  THE  PRINCIPLES  OF  ARROGRAPHY 

1.8  kilograms  per  square  meter  per  year  (5,157  tons  per  square 
mile).  Fry,  at  Cincinnati,  in  work  done  for  the  Smoke 
Enormous  Abatement  League,  collected  dust  in  buckets 
quantities  kept  partly  filled  with  water  and  placed  on  the 
roofs  of  various  buildings.  The  dust  was  filtered 
off,  extracted  with  concentrated  hydrochloric  acid,  and 
weighed.  The  average  monthly  amount  was  60.2  grams  per 
square  meter,  or  723  grams  per  square  meter  for  the  year 
(2,062  tons  per  square  mile).  The  material  collected  was  car- 
bonaceous and  ashy,  probably  derived  largely  from  coal  smoke. 
Between  March  9  and  12,  1901,  dust  fell  at  various  places 
in  Europe,  and  an  effort  was  made  by  Hellmann  and  Meinardus 
African  dust  ^o  estimate  the  quantity.  The  amounts  varied 
in  southern  from  11.23  grams  to  1  gram  per  square  meter  (31 
urope  to  3  tons  per  square  mile).  The  values  were 

largest  in  southern  Europe,  decreasing  toward  the  north. 
The  area  covered  was  at  least  300,000  square  miles  of  land 
surface  and  170,000  square  miles  of  ocean.  The  estimates 
are  somewhat  doubtful,  but  it  is  plain  that  the  total  quantity 
of  sirocco  dust  which  falls  in  Europe  is  very  large.  The 
authors  are  of  the  opinion  that  if  a  storm  as  violent  as  that 
of  March,  1901,  occurred  once  in  five  years,  there  would  have 
been  143  centimeters  of  desert  material  carried  into  Europe 
in  three  thousand  years. 

Measurements  of  the  monthly  deposit  of  soot  in  Pittsburgh, 
during  the  year  ending  March,  1913,  showed  that  at  the 
Pittsburgh's  point  of  maximum  deposit  the  annual  rate  was 
annual  dust  nearly  4  kilograms  per  square  meter  (1,950  tons 
deposit  per  Square  m{\e^  and  at  the  place  of  minimum 

deposit  nearly  1  kilogram  per  square  meter  (600  tons  per 
square  mile).  Measurements  made  at  Leeds,  England,  serve 
as  a  basis  for  estimating  that  there  are  annually  introduced 
into  the  air  near  that  city  35,000,000  kilograms  of  soot. 

John  Aitken  of  Edinburgh  has  probably  done  more  than 
any  one  else  in  studying  the  effect  of  dust  as  nuclei  of  cloudy 
condensation.  His  several  papers  in  the  Transactions  of  the 
Royal  Society  of  Edinburgh  show  the  mean  limit  of  visibility 
for  different  winds  and  the  relation  to  the  dust  content. 
By  means  of  a  dust  counter  he  has  shown  that  the  product 


DUST  AND  MICROBES  157 

of  the  number  of  particles  per  unit  space  by  the  distance  or 
limit  of  visibility  is  constant  for  equal  depressions  of  the  wet 
bulb.  It  appears  that  for  a  depression  of  3  degrees,  1,000 
particles  per  cubic  centimeter  would  completely 
obscure  large  objects  at  a  distance  of  100  miles; 
that  100  times  the  number  of  particles  would 
completely  obscure  objects  at  a  distance  of  1  mile;  and  a 
million  particles  would  obscure  objects  at  one  tenth  of  a  mile. 
In  large  cities  the  number  of  particles  may  exceed  300,000 
per  cubic  centimeter  even  in  fine  weather. 

Carl  Barus,  in  his  paper  on  "Condensation  of  Atmospheric 
Vapor,"1  shows  how  cloudy  condensation  depends  on  air 
temperature  and  dust  content.  Aitken  has  shown  that 
condensation  takes  place  only  upon  free  surfaces  Precondition 
when  saturation  temperature  exists  and  only  when  of  conden- 
sufricient  nuclei  are  present.  His  experiments  satlon 
suggested  a  possible  method  of  counting  dust  particles  which, 
for  the  most  part,  are  too  small  to  be  seen  even  with  a  micro- 
scope. By  making  the  extremely  small  particles  as  well  as 
the  larger  ones  centers  of  condensation,  that  is,  making  them 
the  nuclei  of  small  raindrops,  it  is  practicable  to  count  the 
drops  and  determine  the  number  of  particles.  By  mixing  a 
small  quantity  of  dusty  air  with  a  large  quantity  of  dustless 
air,  and  allowing  the  particles  to  fall  on  a  micrometer,  they 
can  be  counted  by  the  aid  of  a  magnifying  glass.  Then, 
knowing  the  proportion  of  dustless  to  dusty  air  and  allowing 
for  dilution,  the  number  of  particles  can  be  estimated.  The 
general  plan  of  Aitken 's  dust  counter  is  shown  in  the  diagram 
(Fig.  63) .  A  is  the  test  receiver  where  the  air  under 
investigation  is  introduced  and  the  particles  are 
counted.  It  is  an  ordinary  glass  flask  with  flat  bottom,  sup- 
ported in  an  inverted  position.  B  is  an  air  pump  connected 
with  A  by  an  India-rubber  tube  C.  The  pump  B  is  drawn 
in  the  position  shown  in  the  diagram  for  convenience  of 
illustration;  in  practice,  it  is  placed  above  the  level  of  the 
table  for  convenience  in  using  the  pump  while  the  eye  is 

1  Weather  Bureau  Bulletin  No.  12,  1893,  and  Smithsonian  Contributions, 
Vol.  XXXIV,  1905.  See  also  The  Nucleation  of  the  Uncontaminated  Atmosphere 
by  Barus  and  Pierce,  Carnegie  Institution,  Jan.,  1906. 


158 


THE  PRINCIPLES  OF  AEROGRAPHY 


watching  at  the  magnifying  glass.     D  'is  a  cotton-wool  filter 
connected  with  A  by  means  of  the  pipe  E.     The  pipes  C  and 


FIG.   63.     THE  ORIGINAL  DUST  COUNTER  (AITKEN) 

E  pass  through  an  India-rubber  stopper  in  A  and  project 
upward  into  the  receiver;  C  stops  about  the  middle,  while 
E  rises  to  near  the  top  and  forms  the  support  to  which  the 
counting  stage  O  is  attached.  F  is  a  stopcock  for  closing  the 
connection  between  the  receiver  A  and  the  filter  D. 


DUST  AND  MICROBES  159 

To  keep  the  air  under  examination  saturated  with  vapor, 
some  water  is  put  in  the  receiver  A ,  and  from  time  to  time  the 
receiver  is  inverted.  The  stage  0,  on  which  the  drops  are 
counted,  is  a  small  plate  of  highly  polished  silver  about  1 
centimeter  square,  ruled  with  fine  lines  at  right  angles  to  each 
other  and  1  millimeter  apart.  The  stage  is  supported  exactly 
1  centimeter  below  the  flat  top  of  the  receiver.  Otherwise  the 
stage  would  become  covered  with  a  heavy  deposit  of  dew, 
when  it  is  of  no  service.  When  mounted  as  shown  the  dew 
is  easily  cleared  away  by  heating  the  tube  E.  The  heat  is 
carried  forward  to  the  stage  by  the  entering  air.  The  stage 
is  not  placed  centrally  over  the  pipe  E,  because  if  the  stage 
is  too  hot  the  drops  roll  away  and  quickly  evaporate;  on  the 
other  hand,  if  they  are  too  cold  the  surface  becomes  wet  and 
counting  is  impossible.  The  stage  is  viewed  through  the 
bottom  of  the  flask,  using  the  magnifying  glass  5.  The 
following  example  shows  the  manner  of  estimating  the  number 
of  particles  in  a  sample  of  air. 

In  addition  to  the  number  of  drops  per  square  millimeter, 
the  quantities  required  to  complete  the  estimate  are  the 
capacities  of  A  and  B.  If  A  has  a  capacity  of  500  cubic 
centimeters  and  there  are  50  cubic  centimeters  of  water,  its 
air  capacity  is  reduced  to  450  cubic  centimeters.  If  into  this 
pure  air  we  introduce  1  cubic  centimeter  of  the  air  to  be 
tested,  the  dusty  air  will  be,  so  to  speak,  diluted  450  times. 
But  the  air  is  not  only  diluted;  it  is  also  expanded  by  the 
pump,  which  has  a  capacity  of  150  cubic  centimeters.  The 
dust  that  was  in  the  original  1  cubic  centimeter  is  thus 
expanded  by  the  two  processes  into  600  cubic  centimeters. 
The  number  of  drops  per  cubic  centimeter  counted  on  the 
stage  must,  therefore,  be  multiplied  by  600  to  give  the  num- 
ber in  the  original  cubic  centimeter  of  dusty  air.  Suppose 
that  we  counted  one  drop  per  square  millimeter,  then  as 
there  is  1  centimeter  of  air  above  the  stage,  this  will  give  100 
drops  per  cubic  centimeter  in  the  diluted  and  expanded  air; 
and  this,  multiplied  by  600,  gives  60,000  dust  particles  per 
cubic  centimeter  of  the  air  tested. 

Aitken  describes  the  many  precautions  necessary  in  using 
the  apparatus  and  the  difficulties  he  met  in  his  investigations. 


160  THE  PRINCIPLES  OF  A&ROGRAPHY 

He  points  out,  too,  that  a  very  slight  degree  of  supersatura- 
tion  will  cause  condensation  of  some  of  the  dust  particles  in 
the  air;  but  that  the  degree  of  supersaturation  which  is  suf- 
ficient to  cause  some  of  the  particles  to  become  active  centers 
is  yet  insufficient  to  cause  condensation  to  take  place  on 
others.  This  might  indicate  that  the  condensing  power  of 
dust  particles  is  affected  by  their  size.  Some  of  the  values 
obtained  by  Aitken  are: 

NUMBER  OF  DUST  PARTICLES  IN  AIR 


Source 

Number  per  c.c. 

Outside  (raining)  
Outside  (fair)  
Room  
Room  (near  ceiling)  .... 
Bunsen  flame  

32,000 
130,000 
1,860,000 
5,420,000 
30,000,000 

These  numbers  are  very  far  from  being  constant,  and 
vary  with  conditions.  In  one  of  his  papers1  Aitken  mentions 
that  a  cigarette  smoker  sends  4,000,000,000  particles,  or 
more,  into  the  air  with  every  puff.  About  the  smallest 
number  of  particles  observed  is  500  per  cubic  centimeter. 
The  following  are  some  of  the  conclusions  arrived  at: 

The  earth's  atmosphere  is  greatly  polluted  with  dust  produced  by 
human  agency.  This  dust  is  carried  to  a  considerable  elevation 
by  the  hot  air  rising  over  cities. 

The  transparency  of  the  air  depends  on  the  number  of  dust  particles 
in  it  and  also  on  its  humidity.  The  less  dust,  the  more  transparent 
the  air;  and  the  dryer  the  air  the  more  transparent  it  is.  There 
is  no  evidence  that  humidity  alone,  that  is,  water  in  its  gaseous 
state  and  apart  from  dust,  has  any  effect  on  the  transparency. 

The  dust  particles  in  the  atmosphere  have  vapor  condensed  on  them 
though  the  air  may  not  be  saturated. 

The  amount  of  vapor  condensed  on  the  dust  in  unsaturated  air 
depends  on  the  relative  humidity  and  also  on  the  absolute  humid- 
ity. The  higher  the  humidity  and  the  higher  the  vapor  tension, 
the  greater  is  the  amount  of  moisture  held  by  the  dust  particles 
when  the  air  is  not  saturated. 

Haze  is  generally  produced  by  dust;  and  if  the  air  be  dry  the  vapor 
has  little  effect  and  the  density  of  the  haze  depends  chiefly  on 
the  number  of  particles  present. 

i  Royal  Soc.  of  Edin.,  Proceedings,  Feb.  4,  1889. 


DUST  AND  MICROBES  161 

Barus  and  Pierce  have  made  many  measurements  of  atmos- 
pheric nucleation.  The  details  of  these  experiments  cannot 
be  given  here;  but  the  general  deductions  are  of 
much  interest,  such  as  the  extremely  high  nuclea- 
tion  found  in  winter  months  as  compared  with 
summer  months;  again  the  efficiency  of  rain  in  depressing 
nucleation;  and  finally  the  totally  different  character  of  the 
curves  in  different  years.  Simultaneous  measurements  made 
at  Providence  and  at  Block  Island  show  that  the  nucleations 
at  the  former  station  are  much  in  excess,  perhaps  as  much  as 
fifty  times  greater.  The  difference  may  be  due,  since  it  is 
exaggerated  in  the  winter  months,  to  the  originally  ionized 
products  of  combustion. 

Aitken    devised  what  he  called  a   "koniscope,"   in  which 
the  color  of  transmitted  light  is  determined  by  the  size  of 

the'    cloud    particles,   the    depth    of    color    indi-  _. 

*i  i_  r  Ji  o-t.      Thekomscope 

eating    the    number    of    particles    present.     The 

indications  of  the  koniscope  have  been  compared  with  the 
number  given  by  the  dust  counter  as  follows: 


Dust  counter  particles  per  c.c. 

Koniscope,  depth  of  color 

50000 

Color  just  visible 

80000 

Very  pale  blue 

500,000  
1,500,000  . 

Pale  blue 
Fine  blue 

2,500,000  
4,000,000  

Deep  blue 
Very  deep  blue 

There  is  great  diversity  in  the  action  of  different  kinds  of 
dust  particles  in  producing  fogs.  In  ordinary  country  fogs 
the  dust  particles  are  similar  to  those  in  clouds.  . 

If   the   condensation   be   made   quickly,    then   a       action8©? " 

process  of  differentiation  takes  place,  the  smaller       cloud 

, .  1  ,.  1  ,1      i  •  particles 

particles  evaporating  and  the  larger  ones  increas- 
ing in  size.     In  this  way  clouds  and  ordinary  fogs  tend  to 
rain  themselves  out  of  existence.     Not  so  a  town  fog  in  which 
many    of    the    particles    forming    the    nuclei    of        Dust  with 
condensation  have  an  affinity  for  water.     Such         affinity  for 
affinity  is  fatal  to  differentiation  of  the  particles, 
for  by  checking  the  same  it  prevents  the  natural  decay  and 

12 


162  THE  PRINCIPLES  OF  A&ROGRAPHY 

falling  of  the  water  particles.  Air  with  dust  particles  having 
an  affinity  for  water  tends  to  produce  a  maximum  number  of 
small  water  particles  with  but  little  tendency  to  fall ;  whereas,  if 
there  be  no  affinity,  the  tendency  is  to  produce  a  minimum 
number  of  large  particles  with  a  tendency  to  fall.  The  one 
kind  of  dust  particle  forms  a  persisting  fog,  while  the  other 
forms  a  fog  with  a  tendency  to  rain  itself  away.  This  differ- 
ence seems  to  account  for  the  greater  thickness 
fogsypersist  anc^  persistence  of  town  fogs.  There  is  consider- 
able sulphur  dioxide  in  city  air,  probably  result- 
ing from  the  burning  of  coal;  and  this  serves  to  increase  the 
formation  and  maintenance  of  fog.  Aitken,  in  his  paper  on 
the  sun  as  a  fog  producer,  shows  that  under  the  influence  of 
sunshine  nuclei  are  formed  which  have  such  an  affinity  for 
water  that  condensation  sets  in  at  temperatures  above  the 
saturation  temperature.  Sulphur  dioxide,  while  kept  in  pure 
air,  shows  little  tendency  to  produce  nuclei,  but  combines 
readily  with  other  products  of  combustion  and  then,  as 
Aitken  puts  it,  "falls  from  its  high  state  of  a  free-moving 
gaseous  molecule  to  the  condition  of  a  solid  or  liquid  particle 
confined  to  Brownian  movements;  and  probably  finds  its 
independent  existence  in  a  fog  particle  or  possibly  in  a 
raindrop." 

It  must  be  pointed  out  that,  while  there  may  be  several 
million  dust  particles  in  a  cubic  centimeter  of  air,  small  as 
Gaseous  and  these  are  they  are  not  to  be  confused  with  gaseous 
dust  particles  particles.  The  number  of  molecules  in  a  cubic 
centimeter  of  a  gas  under  standard  conditions  is 
2.7  billion  billion,  or,  as  it  is  generally  expressed,  2.704X  1018. 
To  get  a  better  idea  of  the  size  of  atoms  and  molecules  it  may 
be  said  that  it  requires  11,200  cubic  centimeters  of  hydrogen 
to  weigh  one  gram,  and  in  that  gram  there  would  be  over  300 
thousand  billion  billion  molecules  and  twice  as  many  atoms, 
that  is,  6.05X1023.  The  energy  carried  by  a  beam  of  sunlight 
varies  from  day  to  day  with  the  clearness  of  the  air  and  the 
altitude  of  the  sun.  Atmospheric  transmission  is  much 
dimished  by  perceptible  haziness.  Even  in  clearest  weather 
and  at  high  mountain  stations  there  is  an  appreciable  loss  of 
energy  in  the  passage  through  the  air,  and  the  percentage  of 


DUST  AND  MICROBES  163 

loss  increases   steadily  as  the  wave  length  of  the  radiation 
becomes  smaller.     At   Mount  Wilson,  under  favorable  con- 
ditions, almost  99  per  cent  of  the  infra-red  radia-     Dust  and 
tion — the  long  waves — gets  through;  in  the  green     light 
part  of  the  spectrum  about  90  per  cent  reaches  the      ransmissl(> 
earth;  in  the  violet,  80  per  cent;  and  in  the  ultra-violet,  about 
60  per  cent.     In  certain  parts  of  the  infra-red  the  atmosphere 
is  almost  impervious  to  solar  radiation.     Aside  from  the  actual 
absorption  of  the  energy  by  the  water  vapor,  there  is  a  scat- 
tering of  the  radiation  in  directions  other  than  that  of  ^  the 
direct  ray.     If  this  did  not  occur,  the  sky  on  a  clear  day 
would  appear  black  except  for  whatever  whitish  dust  haze 
might    be    present.     Lord    Rayleigh    has    shown 
that  the  blue  color  of  the  sky  is  the  result  of  this 
irregular  dispersion,  or  scattering,  of  light,  even 
in  dust-free  air,    by  the  gas  molecules.     The  molecules   act 
with  greatest  effect  on  the  short  wave  length,  and  therefore 
the  scattering  power  for  the   violet   and   ultra-violet  waves 
is  ten  times  greater  than  for  red  light;  hence  the  larger  diffu- 
sion   of   blue.     Dust   particles,    on    the   other   hand,    reflect 
light  of  all  colors  equally,  and  when  the  air  is  full  of  dust  we 
have  a  mixed  white  and  blue  sky,  or,  as  it  is  called,  milky 
white.     When  the  sun   is  near  the  horizon  the 
beam  has  a  much  longer  column  of   air  to  tra-     are  r^11861 
verse,  and    the    blue    waves  are    less    effective; 
then    the    sun    looks    orange    or    red.     Furthermore,    when 
the  air  is   full  of  fine  volcanic   dust,   such   as  followed   the 
eruption    of    Krakatau,   August    27,    1883;    or    of    Katmai, 
June  6,  1912;  or  of  La  Soufriere  and  Pelee  in  1902;  or  of  Taal, 
January  27,   1911,  then  there  is  a  marked  absorption,  from 
five  to  ten  times  the  normal  amount,  and  a  marked  coloring 
of  the  sky  at  sunset  and  sunrise.     In  the  eruption  of  Krakatau 
large  quantities  of  dust  were  thrown  high  in  the  air  over 
the    Sunda   Strait,  and   this   was   carried  slowly 
westward  around  the  world,   causing  noticeably    y0icanic  dust 
red  sunsets  for  a  period  of  more  than  twenty 
months  in  temperate  latitudes.     There  were  also  halo  effects 
and  the  so-called  Bishop's  ring  (named  after  the  observer  at 
Honolulu"* .     These  last  were  due  to  diffraction  of  the  light 


164  THE  PRINCIPLES  OF  AEROGRAPHY 

in  passing  through  the  dust.  Abbot,  Fowle,  Kimball,  and 
others  have  studied  these  relations. 

One  source  of  atmospheric  dust  is  the  spray  blown  inland 
from  the  seas,  containing  the  sodium  chloride  known  to  be 
present  in  certain  quantity  in  rain.  The  amount  of  sodium 
Salt  content  chloride  decreases  with  distance  from  the  sea- 
of  the  shore ;  Du  Bois  gives  yearly  averages  varying 

atmosphere        frQm  Q  66  tQ  3Q  milligrams  per  Hter  of  rain       He 

calculates  the  annual  amount  deposited  on  the  dunes  of 
Holland  to  be  at  least  6  million  kilograms  (13,227,720  pounds). 
The  mean  proportion  of  sodium  chloride  in  rain  in  England 
is  2.2  milligrams  per  liter;  at  Rothamsted  it  is  2.01  milligrams 
per  liter;  at  Nantes,  France,  it  is  14  milligrams;  and  at  Troy, 
New  York,  2.7  milligrams. 

OPTICAL    PHENOMENA 

45.  Halos  and  coronas.  Halos  are  effects  caused  by  the 
refraction  and  reflection  of  the  rays  of  the  sun  or  moon  by 
ice  crystals.  There  are  many  different  types  of  halo,  no 
fewer  than  fourteen  having  been  enumerated  by  Hastings. 
The  commonest  form  is  the  halo  of  22°.  There  is  a  close 
relation  between  this  halo  and  the  occurrence  of  cirro-stratus 
cloud.  It  is  a  luminous  ring  with  the  inner  edge  sharply 
defined  and  showing  red.  Proceeding  outward, 
th°e°halo  one  may  detect  orange,  yellow,  and  sometimes 

green,  but  seldom  if  ever  violet.  The  sun  (or 
moon)  is  the  center  of  the  circle;  and  usually  one  sees  only 
the  whitish  ring  with  an  inner  edge  of  brownish  red.  Halos 
may  last  for  several  hours,  according  to  the  duration  and 
thickness  of  the  cirro-stratus  cloud  sheet.  They  are  good  indi- 
cators of  coming  storm  conditions,  for  the  reason 
precipitation  t^iat  tney  depend  upon  the  prevalence  and  inten- 
sity of  the  upper  cloud  layer.  The  frequency  of 
halos  and  subsequent  precipitation  has  been  studied  for  Blue 
Hill  Observatory  by  Palmer.1  A  detailed  description  of  the 
different  forms  of  halos  by  Besson  was  published  in  191 1.2 

1  Monthly  Weather  Review,  July,  1914. 

2  A  translation  is  given  in  the  Monthly  Weather  Review,  July,  1914,  p.  436. 
See  also  same  author  on  the  Halos  of  Nov.  1-2,  1913,  p.  431. 


DUST  AND  MICROBES  165 

Coronas  are  diffraction  phenomena,  frequently  seen  about 

the  moon  closely  surrounding  the  source  of  light. 

™,  u  ,1  j  -r  Corona 

I  hey  are  seldom  more  than  two  degrees  in  radius ; 

and  when  showing  prismatic  colors  the  red  is  at  the  outer 
edge.  Solar  coronas  due  to  passing  clouds  must  not  be  con- 
fused with  the  solar  corona  proper. 

Parhelia  (mock  suns)  are  luminous  spots  about  22  degrees 
distant  from  the  sun.     Paraselenae  (mock  moons)  are  similar 
whitish  spots  produced  by  clouds  passing  before 
the   moon.     They  have  the  usual  red  coloring  on 
the  edge  nearest  the  source  of  light.     When  near  the  horizon 
they  are  elongated  vertically,  and  thus  distorted  they  may 
be  mistaken  for  fragments  of  a  rainbow.     Par- 
helia  disappear  when  the  solar  altitude  exceeds 
51  degrees.     There  is  a  well-known  type  of  halo  with  a  radius 
of  46  degrees.     When  a  cloud  that  has  caused  a  parhelion 
passes   to  46   degrees   above   the   sun   a   circumzenithal   arc 
is  formed. 

The   anthelion    (counter   sun)    is   a   round,    luminous   spot 
180  degrees  from  the  sun.     It  must  not  be  con- 
founded  with  the  antisolar  corona,  or  "glory"  of 
the  aeronaut. 

A  light  pillar  is  a  train  of  light  extending  vertically  above 
the  sun  or  moon;  and  it  may  also  be  prolonged  beneath  the 
luminary.  It  must  not  be  mistaken  for  the  luminous  rays 
that  diverge  in  all  directions  when  sunlight  is  streaming 
through  breaks  in  the  clouds.  The  light  pillar  is  always 
vertical. 

It  is  perhaps  proper  to  remark  that  a  halo  described  by 
Bravais  as  the  most  authentic  of  all  extraordinary  halos,— 
the  90-degree  halo  of  Helvelius  (reported  as  seen  Faulty  data 
in  1661),- — does  not  really  exist,  according  to  regarding 
investigations  made  by  Hastings.  This  writer 
insists,  moreover,  that  in  explaining  the  various  halo  forms 
only  such  forms  of  ice  crystals  as  are  known  to  exist  can  be 
taken  account  of;  further,  that  the  orientation  of  the  falling 
crystals  must  conform  to  the  law  of  mechanics;  and,  finally, 
that  all  those  features  of  halos  attributable  to  reflections  must 
find  their  explanation  in  every  case  in  total  reflections.  Much, 


166  THE  PRINCIPLES  OF  A&ROGRAPHY 

therefore,  that  has  been  advanced  regarding  halos  is  still 
unproved. 

In  foggy  weather  an  observer,  especially  if  he  is  on  a  height 

standing  with  his  back  to  the  sun,  will  see  the  shadow  of  his 

head  or  body  cast  upon  the  fog,  surrounded  by  a 

"Wsation"        colored  ring  of  light,   variously   called   "glory," 

"Ulloa's  ring,"    "Brocken  specter."     The  green 

and  red  patches  occasionally  seen  in  cirrus  clouds  are  known 

as  "irisation." 

The  colors  of  rainbows,  as  well  as  the  extent  and  position 
of  greatest  luminosity,  depend  upon  the  size  of  the  drops 

producing  the  bow.     It  has  been  erroneously  as- 
Rambows  1  .-,     ^     n       •  V.  ^ 

sumed  that  all  rainbows  show  the  same  sequence 

of  colors  and  have  the  same  radius.  If  the  sequence  of 
colors  in  the  primary  bow  commencing  with  the  red  is  noticed, 
also  the  color  showing  maximum  luminosity  and  which 
color  band  is  widest,  the  size  of  the  drop  can  be  determined. 
Light  is  both  reflected  and  refracted  at  the  surface  boundary 
of  air  and  water;  and  a  rainbow  is,  therefore,  the  result  of 
reflection,  refraction,  and  interference  combined.  Since  each 
illuminated  point  on  a  drop  of  water  in  midair  receives  light 
from  the  whole  disk  of  the  sun,  there  are  many  overlapping 
spectra.  The  width  and  degree  of  separation  of  the  inter- 
ference bands  is  increased  as  the  size  of  the  drops  diminishes. 
According  to  Pernter,  rainbows  of  richest  color  are  produced 
by  drops  varying  from  0.2  to  0.4  millimeter  in  diameter. 


CHAPTER   XIV 

ATMOSPHERIC  ELECTRICITY 

46.  Thunderstorm  phenomena.  There  are  few  more  pro- 
nounced manifestations  of  atmospheric  motion  than  a  well- 
developed  thunderstorm.  While  the  lightning  discharges  are 
perhaps  the  most  spectacular  feature,  there  is  much  more  to 
a  storm  of  this  character  than  the  electrical  display;  and, 
indeed,  it  is  doubtful  if  even  what  we  see  of  the  lightning  is 
more  than  a  small  part  of  the  phenomena  and  the  conversion 
and  dissipation  of  energy. 

The  identification  of  lightning  with  electricity  was  the 
work  of  Franklin,  and  the  classical  kite  experiment  is 
described  by  him  in  a  letter  dated  October  19, 
1752  (old  style).  Very  few  investigators  have  experiment 
repeated  the  experiment  as  described  by  Franklin, 
and,  as  a  matter  of  fact,  the  results  are  somewhat  different 
from  those  he  described.1  He  did  not  draw  the  lightning 
from  the  clouds,  as  is  so  generally  stated,  but  did  obtain 
moderate  induction  effects ;  and  these  can  be  obtained  without 
much  danger  on  the  approach  of  heavily  charged  clouds ;  but 
any  direct  discharge  or  flash  of  lightning  will  demolish  both 
kite  and  string  and  probably  injure  the  observer.  Thus  at 
Blue  Hill  Observatory  during  a  kite  flight,  March  6,  1913, 
while  there  had  been  the  usual  static  discharges,  there  was 
no  lightning  and  no  thunder  previous  to  a  discharge  at 
12:41  P.M.,  when  1,500  meters  of  steel  wire  were  volatilized, 
and  the  observers  stunned. 

i  Rotch,  in  Science,  Dec.  14,  1906,  shows  that  Franklin  did  not  fly  his  kite 
until  later  in  summer  than  June,  1752,  or  some  two  months  later  than  has 
generally  been  supposed.  Franklin  had  already  prepared  for  publication  pre- 
cise directions  for  placing  lightning  rods  upon  all  kinds  of  buildings.  The 
letter  to  Collinson  is  dated  Oct.  19,  1752.  It  reads: 

"Make  a  small  cross  of  light  sticks  of  cedar,  the  arms  so  long  as  to  reach 
to  the  four  corners  of  a  large  thin  silk  handkerchief  when  extended.  Tie  the 
corners  of  the  handkerchief  to  the  extremities  of  the  cross,  so  you  have  the 
body  of  a  kite,  which  being  properly  accommodated  with  a  tail,  loop  and  string, 
will  rise  in  the  air  like  those  made  of  paper,  but  being  made  of  silk  is  better 
fitted  to  bear  the  wet  and  wind  of  a  thunder  gust  without  tearing.  To  the 

167 


168  THE  PRINCIPLES  OF  AEROGRAPHY 

An  excellent  resume  of  recent  investigation  of  thunderstorm 
phenomena  is  given  by  Humphreys  in  the  Monthly  Weather 
Review,  June,  1914,  from  which  the  following  extract  is  taken: 

"A  thunderstorm  is  a  storm  characterized  by  thunder  and 
lightning,  just  as  a  dust  storm  is  one  characterized  by  a  great 
quantity  of  flying  dust.  But  the  dust  is  never  in  any  sense 
the  cause  of  the  storm  that  carries  it  along,  nor,  so  far  as 
known,  does  either  thunder  or  lightning  have  any  influence 
on  the  course — genesis,  development,  or  termination — of 
even  those  storms  of  which  they  form,  in  some  respects,  the 
most  important  features.  No  matter  how  impressive  or  how 
terrifying  these  phenomena  may  be,  they  never  are  anything 
more  than  mere  incidents  to  or  products  of  the  peculiar 
storms  they  accompany,  as  will  be  made  clear  by  what  follows. 
In  short,  they  are  never  in  any  sense  either  storm-originating 
or  storm-controlling  factors. 

"A  knowledge,  or  at  least  a  good  working  hypothesis,  of  how 
the  great  amount  of  electricity  incident  to  thunderstorms  is 
Origin  of  generated,  is  absolutely  essential  to  their  logical 
atmospheric  explanation;  that  is,  to  a  clear  understanding  of 
the  probable  interrelations  between  their  many 
phenomena.  Fortunately  such  an  hypothesis,  or  theory 
rather,  since  it  is  abundantly  supported  by  observations  and 
by  laboratory  experiments,  is  available  as  a  result  of  work 
done  on  this  subject  in  India  by  Dr.  G.  C.  Simpson1  of  the 
Indian  Meteorological  Department. 

"Dr.  Simpson's  observations  were  obtained  at  Simla, 
India,  at  an  elevation  of  about  7,000  feet  above  sea  level, 
and  covered  all  of  the  monsoon  seasons,  that  is,  roughly, 

top  of  the  upright  stick  of  the  cross  is  to  be  fixed  a  sharp  pointed  wire  rising 
a  foot  or  more  above  the  wood.  To  the  end  of  the  twine  next  the  hand  is  to 
be  tied  a  silk  ribbon  and  where  the  silk  and  twine  join  a  key  may  be  fastened. 
This  kite  is  to  be  raised  when  a  thunder  gust  appears  to  be  coming  on,  and  the 
person  who  holds  the  string  must  stand  within  a  door  or  window  or  under  some 
cover  so  that  the  silk  ribbon  may  not  be  wet:  and  care  must  be  taken  that  the 
twine  does  not  touch  the  frame  of  the  door  or  window.  As  soon  as  the  thunder 
clouds  come  over  the  kite  the  pointed  wire  will  draw  the  electric  fire  from  them 
and  the  kite  with  all  the  twine  will  be  electrified,  and  stand  out  every  way 
and  be  attracted  by  an  approaching  finger.  And  when  the  rain  has  wet  the 
kite  and  twine  you  will  find  the  electric  fire  stream  out  plentifully  from  the 
key  on  the  approach  of  your  knuckle." 

1  Memoirs,  Indian  Met.  DepL,  Simla,  Vol.  XX  (1910),  Pt.  8. 


ATMOSPHERIC  ELECTRICITY  169 

April  15  to  September  15,  of  1908  and  1909.  He  also  obtained 
observations  of  the  electrical  conditions  of  the  snow  at  Simla 
during  the  winter  of  1908-1909. 

"A  tipping-bucket  rain  gauge  gave  an  automatic  con- 
tinuous record  of  the  rate  and  time  of  rainfall,  while  a  Benndorf 
self -registering  electrometer  marked  the  sign  and  potential  of 
the  charge  acquired  during  each  two-minute  interval.  A 
second  Benndorf  electrometer  registered  the  potential  gradient 
near  the  earth,  and  a  coherer  of  the  type  used  in  radio  teleg- 
raphy registered  the  occurrence  of  each  lightning  discharge. 

"All  obvious  sources  of  error  were  examined  and  carefully 
guarded  against.     Hence  it  would  seem  that  the  conclusions 
drawn  from  the  thousands  of  observations  given 
in  the  memoir  are  fully  justified ;  and  especially         records0  * 
so  since  several  independent  series  of  similar  ob- 
servations made  at  different  times,  by  different  people  and  at 
places  widely  separated,  have  given  confirmatory  results  in 
every  case.     Simpson's  records  show  that  — 

"(1)  The  electricity  brought  down  by  the  rain  was  sometimes 
positive  and  sometimes  negative. 

"  (2)  The  total  quantity  of  positive  electricity  brought  down  by  the 
rain  was  3.2  times  greater  than  the  total  quantity  of  negative  elec- 
tricity. 

"  (3)  The  period  during  which  positively  charged  rain  fell  was  2.5 
times  longer  than  the  period  during  which  negatively  charged  rain 
fell. 

"(4)  Treating 'charged  rain  as  equivalent  to  a  vertical  current  of 
electricity,  the  current  densities  were  generally  smaller  than  4  X  10~15 
amperes  per  square  centimeter ;  but  on  a  few  occasions  greater  current 
densities,  both  positive  and  negative,  were  recorded. 

"(5)  Negative  currents  occurred  less  frequently  than  positive 
currents,  and  the  greater  the  current  density  the  greater  the  pre- 
ponderance of  the  positive  currents. 

' '  (6)  The  charge  carried  by  the  rain  was  generally  less  than  6 
electrostatic  units  per  cubic  centimeter  of  water,  but  larger  charges 
were  occasionally  recorded,  and  in  one  exceptional  storm  (May  13, 
1908)  the  negative  charge  exceeded  19  electrostatic  units  per  cubic 
centimeter. 

"'(7)  As  stated  in  paragraph  (3)  above,  positive  electricity  was 
recorded  more  frequently  than  negative,  but  the  excess  was  the  less 
marked  the  higher  the  charge  on  the  rain. 

"  (8)  With  all  rates  of  rainfall  positively  charged  rain  occurred  more 
frequently  t*han  negatively  charged  rain,  and  the  relative  frequency  of 


170  THE  PRINCIPLES  OF  AEROGRAPHY 

positively  charged  rain  increased  rapidly  with  increased  rate  of  rain- 
fall. With  rainfall  of  less  than  about  1  millimeter  in  two  minutes, 
positively  charged  rain  occurred  twice  as  often  as  negatively  charged 
rain,  while  with  greater  intensities  it  occurred  14  times  as  often. 

"  (9)  When  the  rain  was  falling  at  a  less  rate  than  about  0.6  milli- 
meter in  two  minutes,  the  charge  per  cubic  centimeter  of  water 
decreased  as  the  intensity  of  the  rain  increased. 

"  (10)  With  rainfall  of  greater  intensity  than  about  0.6  millimeter 
in  two  minutes  the  positive  charge  carried  per  cubic  centimeter  of 
water  was  independent  of  the  rate  of  rainfall,  while  the  negative 
charge  carried  decreased  as  the  rate  of  rainfall  increased. 

"(11)  During  periods  of  rainfall  the  potential  gradient  was  more 
often  negative  than  positive,  but  there  were  no  clear  indications  of  a 
relationship  between  the  sign  of  the  charge  on  the  rain  and  the  sign 
of  the  potential  gradient. 

"  (12)  The  data  do  not  suggest  that  the  negative  electricity  occurs 
more  frequently  during  any  particular  period  of  a  storm  than  during 
any  other. 

"Concerning  his  observation  on  the  electrification  of  snow 
Dr.  Simpson  says: 

"  'As  far  as  can  be  judged  from  the  few  measurements  made  during 
the  winter  of  1908-1909  it  would  appear  that: 

'"(1)  More  positive  than  negative  electricity  is  brought  down  by 
snow  in  the  proportion  of  about  3.6  to  1. 

'"(2)  Positively  charged  snow  falls  more  often  than  negatively 
charged. 

'"(3)  The  vertical  electric  currents  during  snowstorms  are  on  the 
average  larger  than  during  rainfall. 

'"(4)  The  charge  per  unit  mass  of  precipitation  is  larger  during 
snowfall  than  during  rainfall.' 

"While  these  observations  were  being  secured,  a  number 
of  well-devised  experiments  were  made  to  determine  the 
electrical  effects  of  each  obvious  process  that  takes  place 
in  the  thunderstorm. 

"Freezing  and  thawing,  air  friction,  and  other  things  were 
tried,  but  none  produced  any  electrification.  Finally,  on 
allowing  drops  of  distilled  water  to  fall  through  a  vertical 
blast  of  air  of  sufficient  strength  to  produce  some  spray, 
positive  and  important  results  were  found,  showing: 

"(1)  That  breaking  of  drops  of  water  is  accompanied  by  the 
production  of  both  positive  and  negative  ions. 

"(2)  That  three  times  as  many  negative  ions  as  positive  ions  are 
released. 


ATMOSPHERIC  ELECTRICITY  171 

"Now,  a  strong  upward  current  of  air  is  one  of  the  most 
conspicuous  features  of  the  thunderstorm.  It  is  always 
evident  in  the  turbulent  cauliflower  heads  of  the  cumulus 
cloud,  the  parent,  presumably,  of  all  thunderstorms.  Besides, 
its  inference  is  compelled  by  the  occurrence  of  hail,  a  frequent 
thunderstorm  phenomenon,  whose  formation  requires  the 
carrying  of  raindrops  and  the  growing  hailstones  repeatedly 
to  cold  and  therefore  high  altitudes.  And  from  the  existence 
of  hail  it  is  further  inferred  that  an  updraft  of  at  least  8  meters 
per  second  must  often  occur  within  the  body  of  the  storm, 
since,  as  experiment  shows,  it  requires  approximately  this 
velocity  to  support  the  larger  drops,  and  even  a  greater 
velocity  to  support  the  average  hailstone. 

"Experiment  also  shows  that  rain  cannot  fall  through  air 
of  ordinary  density  whose  upward  velocity  is  greater  than 
about  8  meters  per  second,  or  itself  fall  with  greater  velocity 
through  still  air;  that  in  such  a  current,  or  with  such  a  veloc- 
ity, drops  large  enough,  if  kept  intact,  to  force  their  way 
down,  or,  through  the  action  of  gravity,  to  attain  a  greater 
velocity  than  8  meters  per  second  with  reference  to  the  air, 
whether  still  or  in  motion,  are  so  blown  to  pieces  that  the 
increased  ratio  of  supporting  area  to  total  mass  causes  the 
resulting  spray  to  be  carried  aloft  or  left  behind,  together 
with,  of  course,  all  original  smaller  drops.  Clearly,  then,  the 
updrafts  within  a  cumulus  cloud  frequently  must 
break  up  at  about  the  same  level  innumerable  {he^mg~  r°p 
drops  which,  through  coalescence  have  grown 
beyond  the  critical  size,  and  thereby,  according  to  Simpson's 
experiments,  produce  electrical  separation  within  the  cloud 
itself.  Obviously,  under  the  turmoil  of  a  thunderstorm,  its 
choppy  surges  and  pulses,  such  drops  may  be  forced  through 
the  cycle  of  union  (facilitated  by  any  charges  they  may  carry) 
and  division,  of  coalescence  and  disruption,  from  one  to  many 
times,  with  the  formation  on  each  at  every  disruption,  again 
according  to  experiment  of  a  correspondingly  increased  electrical 
charge.  The  turmoil  compels  mechanical  contact  between  the 
drops,  whereupon  the  charges  break  down  the  surface  tension 
and  insure  coalescence.  Hence,  once  started,  the  electricity 
of  a  thunderstorm  rapidly  grows  to  a  considerable  maximum. 


172  THE  PRINCIPLES  OF  AgROGRAPHY 

"After  a  time  the  larger  drops  reach,  here  and  there,  places 
below  which  the  updraft  is  small — the  air  cannot  be  rushing 
up  everywhere — and  then  fall  as  positively  charged  rain, 
because  of  the  processes  just  explained.  The  negative  elec- 
trons in  the  meantime  are  carried  up  into  the  higher  portions 
of  the  cumulus,  where  they  unite  with  the  cloud  particles 
and  thereby  facilitate  their  coalescence  into  negatively  charged 
drops.  Hence,  the  heavy  rain  of  a  thunderstorm  should  be 
positively  charged,  as  it  almost  always  is,  and  the  gentler 
portions  negatively  charged,  which  very  frequently  is  the  case. 

"Such,  in  brief,  is  Dr.  Simpson's  theory  of  the  origin  of  the 
electricity  in  thunderstorms,  a  theory  that  fully  accounts  for 
the  facts  of  observation  and  in  turn  is  itself  abundantly  sup- 
ported by  laboratory  tests  and  simulative  experiments. 

"If  this  theory  is  correct,  and  it  seems  well  founded,  it 
must  follow  that  the  one  essential  to  the  formation  of  the 
giant  cumulus  cloud,  namely,  the  rapid  uprush  of  moist  air, 
is  also  the  one  essential  to  the  generation  of  the  electricity 
of  thunderstorms.  Hence  the  reason  why  lightning  seldom, 
if  ever,  occurs  except  in  connection  with  a  cumulus  cloud  is 
understandable  and  obvious.  It  is  simply  because  the  only 
process  that  can  produce  the  one  is  also  the  process  that  is 
necessary  and  sufficient  for  the  production  of  the  other. 

"The  violent  motions  of  cumulus  clouds.  From  observa- 
tions, and  from  the  graphic  descriptions  of  the  few  balloonist s 
who  have  experienced  the  trying  ordeal  of  passing  through 
the  heart  of  a  thunderstorm,  it  is  known  that  there  is  violent 
vertical  motion  and  much  turbulence  in  the  middle  of  a  large 
cumulus  cloud,  a  fact  which,  so  far  as  it  relates  to  the  theory 
alone  of  the  thunderstorm,  it  would  be  sufficient  to  accept 
without  inquiring  into  its  cause.  However,  to  render  the 
discussion  more  nearly  complete,  it  perhaps  is  worth  while, 
since  it  is  a  moot  question,  to  inquire  what  the  probable 
cause  of  the  violent  motions  in  large  cumulus  clouds  really 
is — motions  which,  in  the  magnitude  of  their  vertical  com- 
ponents and  degree  of  turmoil,  are  never  exhibited  by  clouds 
of  any  other  kind  nor  met  with  elsewhere  by  either  manned, 
sounding,  or  pilot  balloons.1" 

1  Simpson's  views  are  given  at  length  in  a  paper  on  "  The  Electricity  of 
Atmospheric  Precipitation,"  Phil.  Mag.,  S.  6,  Vol.  XXX  (1915),  p.  1. 


ATMOSPHERIC  ELECTRICITY 


173 


47.  Cause  of  the  turbulence  in  thunderstorms.  Hum- 
phreys thinks  that  the  difference  between  the  actual  tempera- 
ture gradient  of  the  surrounding  atmosphere  and  the  gradient 
(adiabatic)  for  saturated  air  within  the  cloud  itself  is  the  cause 
of  the  turbulence.  Assume  the  temperature  to  be  303°A.  and 
the  dew  point  288°A.  The  adiabatic  decrease,  as  we  have 
seen,  is  approximately  one  degree  per  hundred  meters;  therefore 
condensation  would  begin  at  a  height  of  1.5  kilometers.  It 

may  be  remarked,  

however,  that  we 
have  no  data  justi- 
fying the  use  of 
this  rate  at  times 
of  thunderstorms. 
The  cloud  particles 
are  carried  up,  and 
at  an  elevation  of 
approximately  half 
a  kilometer  above 
the  plane  of  con- 
densation  the 
drops  lag  or  drop. 


?N43    253      £63     273       £83      295 


Humphreys 

FIG.  64.     TEMPERATURE  GRADIENTS  WITHIN  (CLD) 

AND    WITHOUT    (CKD)    CUMULUS    CLOUDS 


Hence  for  two  rea- 
sons,— because  the 
heat  of  the  con- 
densed water  is  no 
longer  available  to 
the  air  from  which 
it  was  condensed, 
and  because  little 
heat  is  available 
from  further  condensation, —  the  rate  of  decrease  again 
approaches  the  assumed  adiabatic  gradient  for  dry  air. 

In  the  accompanying  diagram  (Fig.  64)  AB  is  the  approxi- 
mate temperature  gradient  for  nonsaturated  air.  GCKDEF  is 
the  supposed  temperature  gradient  before  convection  begins; 
that  is,  the  average  rate  as  deduced  from  many  soundings,  or 
about  0.6  degree  instead  of  0.96,  except  near  the  surface 
where  the  gradient  is  even  less  and  sometimes  negative. 


174  THE  PRINCIPLES  OF  ARROGRAPHY 

To  explain  the  abrupt  temperature  change  Humphreys  says : 

"As  convection  sets  in,  the  temperature  decrease  near  the 
surface  soon  approximates  the  adiabatic  gradient  for  dry  air, 
and  this  condition  extends  gradually  to  greater  altitudes,  till, 
in  the  assumed  case,  condensation  begins  at  the  level  C,  or 
where  the  temperature  is  283°A.  Here  the  temperature 
decrease,  under  the  assumed  conditions,  suddenly  changes 
from  10  degrees  per  kilometer  increase  of  elevation  to  rather 
less  than  half  that  amount,  but  slowly  increases  with  increase 
of  altitude  and  consequent  decrease  of  temperature.  At  some 
level,  as  L,  the  temperature  difference  between  the  rising  and 
the  adjacent  air  is  a  maximum.  At  D  the  temperature  of  the 
rising  air  is  the  same  as  that  of  the  air  adjacent,  but  its 
momentum  presumably  carries  it  on  to  some  such  level  as  H. 
Within  the  rising  column,  then,  the  temperature  gradient  is 
approximately  given  by  ACLDHE,  and  that  of  the  surround- 
ing air  by  ACKDEF. 

"The  cause,  therefore,  of  the  violent  uprush  and  turbulent 
condition  within  large  cumulus  clouds  is,  presumably,  the 
Difference  of  difference  between  the  temperature  of  the  inner 
temperature  or  warmer  portions  of  the  cloud  itself  and  that  of 
in  cloud  tjie  surrounding  atmosphere  at  the  same  level, 

as  indicated  by  their  respective  temperature  gradients  CLD 
and  CKD.  Clearly,  too,  while  some  air  must  flow  into  the 
condensation  column  all  along  its  length,  the  greatest  pressure 
difference,  and,  therefore,  the  greatest  inflow,  obviously  is 
at  its  base.  After  the  rain  has  set  in,  however,  this  basal 
inflow  is  from  immediately  in  front  of  the  storm,  and  necessarily 
so,  as  will  be  explained  later." 

Since  rapid  vertical  convection  of  humid  air  is  essential  to 
the  production  of  the  thundercloud,  the  conditions  under  which 
the  vertical  temperature  gradient  necessary  to  this  convection 
can  be  established  must  be  (1)  strong  surface  heating,  especi- 
ally in  regions  of  light  wind;  (2)  the  overrunning  of  one  layer 
of  air  by  another  at  a  temperature  sufficiently  low  to  induce 
convection ;  and  (3)  the  underrunning  and  consequent  uplift  of 
a  saturated  layer  of  air  by  a  denser  layer. 

The  turbulent  character  of  the  air  motion  and  the  irregularity 
of  cloud  mass  are  shown  in  Figs.  65  and  66. 


FIG.  65.  TURBULENCE  IN  THUNDERSTORM  CLOUDS 


IcAdi 


%| 


FIG.  66.     TURBULENCE  IN  THUNDERSTORM  CLOUDS 
Fig.  66  was  taken  four  minutes  later  than  Fig.  65. 

175 


176  THE  PRINCIPLES  OF  A&ROGRAPHY 

48.  Conditions  favorable  to  thunderstorms.  The  more 
humid  the  air  and  the  more  energetic  the  local  convections 
the  greater  the  frequency  and  intensity  of  thunderstorms. 
The  time  of  maximum  frequency  of  inland  thunderstorms  is 
afternoon,  the  time  when  the  vertical  convection  is  at  a 
maximum.  Over  the  ocean,  because  of  evaporation  and  the 
high  specific  heat  of  water,  the  surface  temperature  rises 
slowly  during  the  day  and  falls  slowly  during  the  night. 
The  diurnal  range  of  temperature  at  the  ocean  surface  is 
much  less  than  that  of  the  air,  and  hence  temperature  gra- 
dients over  the  ocean  favorable  for  rapid  vertical  convection 
are  most  frequent  during  the  early  morning  hours;  therefore 
the  maximum  frequency  of  ocean  thunderstorms  is  between 
midnight  and  4  A.M.,  quite  different  from  the  case  on  land, 
where  the  storm  occurs  in  the  afternoon. 

Just  as  thunderstorms  occur  most  frequently  during  the 
hottest  hours  over  the  land,  so  they  are  most  frequent  in  the 
Time  of  hottest  months  over  the  land.  In  middle  lati- 

greatest  tudes  the  maximum  frequency  occurs  in  June, 

requency  an(^  ^  j^g^-p  latitudes  in  July  or  August.  Over 
the  ocean,  on  the  contrary,  temperature  gradients  favorable 
for  the  genesis  of  thunderstorms  occur  most  frequently  during 
the  winter.  There  also  appears  to  be  a  relation  between  the 
rainfall  and  thunderstorm  frequency,  a  relation  which  might 
be  expected. 

Humphreys  advances  the  view  that,  "omitting  the  effects  of 
radiation,  there  seem  to  be  but  three  possible  ways  by  which 
the  cooling  of  a  thunderstorm  may  be  obtained:  (a)  by  the 
descent  of  originally  potentially  cold  air;  (b)  by  chilling  the 
air  with  the  cold  rain;  (c)  by  evaporation.  Each  of  these  will 
be  considered  separately. 

' '  (a)  Obviously,  no  portion  of  the  upper  air  could  main- 
tain its  position  if  potentially,  even  slightly,  colder  .than 
Descent  of  that  near  the  surface.  If  at  all  potentially  colder 
cold  air  it  would  fall  until  it  itself  became  the  surface  air, 

inadequate  ag  jn(}eed  js  fae  case  jn  ar|  vertical  circulation. 

Hence  the  great  decrease  in  temperature  that  comes  with 
a  thunderstorm  is  not  the  result  of  the  descent  of  a  layer 
of  air  originally  potentially  cold. 


ATMOSPHERIC  ELECTRICITY  111 

"(b)  Let  the  under  surface  of  the  thunderstorm  cloud  be 
1,500  meters  above  the  earth,  and  the  column  of  air  cooled 
by  the  cold  rain  and  its  evaporation  2,000  meters  high.  Let 
the  surface  temperature  be  303°A.,  and  the  temperature 
gradient  before  the  storm  begins  adiabatic  up  to  the  under  - 
cloud  level,  and  let  there  be  a  rainfall  of  20  millimeters. 

"Now,  at  the  temperature  assumed,  a  column  of  air  2,000 
meters  high  whose  cross-section  is  1  square  centimeter  weighs, 
roughly,  210  grams,  and  its  heat  capacity,  therefore,  is  approxi- 
mately that  of  50  grams  of  water.  At  the  top  of  this  column 
the  temperature  can  be,  at  most,  only  about  20  degrees  lower 
than  at  the  bottom,  and  if  the  rain  leaves  the  top  at  this 
temperature  but  reaches  the  earth  7  degrees  colder  than  the 
surface  air  before  the  storm  (temperatures  that  seem  at  least  to 
be  of  the  correct  order)  it  will  have  been  warmed  13  degrees 
during  its  fall  and  the  air  column  cooled  on  the  average  about 
0.5  degree.  But,  as  a  matter  of  fact,  the  air  chillin 
usually  is  cooled  by  from  5  to  10  degrees.  Hence,  due  to 

while  the  temperature  of  the  air   necessarily   is        rainfall 

n  ..  1  inadequate 

reduced  to  some  extent  by  mere  heat  conduction 

to  the  cold  rain,  much  the  greater  portion  of  the  cooling  clearly 
must  have  some  other  origin.  Let  us  see,  then,  if  evaporation 
really  is  adequate  to  meet  these  demands. 

' '  (c)  It  is  a  common  thing  in  semiarid  regions  to  see  a 
heavy  shower,  even  a  thunder  shower,  leave  the  base  of  a 
cloud  and  yet  fail  utterly,  because  of  evaporation,  to  reach 
the  surface  of  the  earth.  Hence  it  appears  quite  certain 
that  in  the  average  thunderstorm  a  considerable 
portion  of  the  rain  that  leaves  the  cloud  is  evap- 
orated  before  it  reaches  the  ground,  and  therefore 
that  the  temperature  decrease  of  the  atmosphere  is  largely 
due  to  this  fact.  But  if  so,  why,  then,  one  might  properly 
ask,  does  not  an  equally  great  temperature  drop  accompany 
all  heavy  rains? 

"The  answer  is  obvious,  because,  as  a  rule,  the  tempera- 
ture is  higher  and  the  relative  humidity  lower  during  a  thunder- 
storm than  at  the  time  of  an  ordinary  rain.  The  chief, 
perhaps  the  sole,  reason  for  this  difference  in  relative  humidity 
is  the  difference  in  the  two  cases  between  the  movements  of 

13 


178  THE  PRINCIPLES  OF  A&ROGRAPHY 

the  air.  In  the  thunderstorm  the  descending  air,  which  can 
be  no  more  than  saturated  at  top,  dynamically  warms  so 
rapidly  and  is  so  continuously  renewed  that  evaporation 
into  it  cannot  keep  pace  with  its  vapor  capacity.  During 
other  rains,  however,  where  there  is  no  atmospheric  descent, 
and  therefore  no  dynamical  heating,  approximate  saturation 
must  soon  obtain;  hence  but  little  further  evaporation  and, 
of  course,  but  little  cooling. 

"But  no  matter  how  nor  to  what  extent  the  details  may 
vary,  it  seems  quite  certain  that  the  cold  rain  of  a  thunder- 
storm and  its  evaporation  together  must  establish  a  local 
downrush  of  cold  air — an  observed  important  and  charac- 
teristic phenomenon,  really  the  immediate  cause  of  the 
vigorous  circulation,  whose  rational  explanation  has  been 
attempted  in  the  past  few  paragraphs. 

"As  the  column  or  sheet  of  cold  air  flows  down  it  main- 
tains in  great  measure  its  original  velocity  and,  therefore,  on 
reaching  the  earth  rushes  forward  in  the  direction  of  the 
storm  movement,  underrunning  and  buoying  up  the  adjacent 
warm  air.  And  this  condition,  largely  due,  as  explained,  to 
condensation  and  evaporation,  once  established  necessarily 
is  self -perpetuating,  so  long  as  the  general  temperature  gradi- 
ent, humidity,  and  wind  direction  are  favorable.  It  must  be 
remembered,  however,  that  thunderstorm  convection,  rising 
air  just  in  front  and  descending  air  with  the  rain,  does  not 
occur  in  a  closed  circuit,  for  the  air  that  goes  up  does  not 
return,  nor  does  the  air  that  comes  down  go  up  again  imme- 
diately; there  simply  is  an  interchange  between  the  surface 
air  in  front  of  the  storm  and  the  upper  air  in  its  rear.  The 
travel  of  the  storm,  by  keeping  up  with  the  under- 
rurmin£  c°ld  current,  just  as  effectually  maintains 
the  temperature  contrast  essential  to  this  open- 
circuit  convection  as  does  continuous  heating  on  one  side  and 
cooling  on  the  other  maintain  the  temperature  contrast  essen- 
tial to  a  closed-circuit  convection. 

"The  movements  of  the  warm  air  in  front  of  the  rain,  the 
lull,  the  inflow,  and  the  updraft  resemble  somewhat  those  of 
a  horizontal  cylinder  resting  on  the  earth  where  the  air 
is  quiet  and  rolling  forward  with  the  speed  of  the  storm. 


ATMOSPHERIC  ELECTRICITY  179 

Similarly,  the  cold  air  in  its  descent  and  forward  rush,  together 
with  the  updraft  of  warm  air,  also  resembles  a  horizontal 
cylinder,  but  one  sliding  on  the  earth  and  turning  in  the 
opposite  direction  from  that  of  the  forward-rolling  or  all- 
warm  cylinder.  In  neither  case,  however,  is  the  analogy 
complete,  for,  as  above  explained,  the  air  that 
goes  up  remains  aloft  while  the  cold  air  that  comes  of°flow°E 
down  is  kept  by  its  greater  density  to  the  lower 
levels.  The  condition  of  flow  persists,  as  do  cataracts  and 
crest  clouds,  but  here,  too,  as  in  their  case,  the  material  involved 
is  ever  renewed. 

"Between  the  uprising  sheet  of  warm  air  and  the  adjacent 
descending  sheet  of  cold  air,  horizontal  vortices  are  sure  to  be 
formed  in  which  the  two  currents  are  more  or  less  mixed.  The 
lower  of  these  vortices  can  only  be  inferred  as  a  necessary 
consequence  of  the  opposite  directions  of  flow  of 
the  adjacent  sheets  of  warm  and  cold  air,  for  there 
is  nothing  to  render  them  visible.  Neither  can 
any  vortices  that  may  exist  within  the  cloud  be  seen.  Near 
the  front  lower  edge  of  the  cumulo-nimbus  system,  however, 
and  immediately  in  front  of  the  sheet  of  rain,  or  rain  and  hail, 
the  rising  air  has  so  nearly  reached  its  dew  point  that  the  some- 
what lower  temperature,  produced  by  the  admixture  of  the 
descending  cold  air,  is  sufficient  to  produce  in  it  a  light  foglike 
condensation  which,  of  course,  renders  any  detached  vortex 
at  this  position  quite  visible. 

"This  squall  cloud,  in  which  the  direction  of  motion  on  top 
is  against  the  storm,  may  be  regarded  as  a  third  horizontal 
thunderstorm  cylinder  much  smaller  but  more  complete  than 
either  of  the  others. 

"The  above  conceptions  of  the  mechanism  of  a  thunder- 
storm can,  perhaps,  be  made  a  little  clearer  with  the  aid  of 
illustrations.     Fig.   67,   a  schematic  picture  of  a 
thunderstorm  in  the  making,  gives  the  boundary    Jhunderstorm 
of  a  large  cumulus  cloud  from  which  rain  has  not 
yet  begun  to  fall,  and  the  stream  lines  of  atmospheric  flow  into 
it.     When  the  cloud  is  stationary  and  there  is  no  surface  wind 
the  updraft  obviously  will  be  more  or  less  symmetrical  about 
a  vertical  through  its  center,  but  when  it  has  an  appreciable 


180 


THE  PRINCIPLES  OF  AEROGRAPHY 


velocity,  as  indicated  in  the  figure,  it  is  equally  obvious  that 
most,  often  nearly  all,  of  the  air  entering  the  cloud  will  do  so 
through  its  front  under-surface.  At  this  stage  there  will  be 
no  concentrated  or  local  down  current,  only  an  imperceptible 
counter  settling  of  the  air  round  about,  because,  as  previously 


FIG.  67.     THUNDERSTORM  IN  THE  MAKING 

explained,  the  air  cataract  requires  local  cooling  to  subpoten- 
tial  temperatures,  and  this  in  turn  requires  local  rain. 

"Fig.  68  schematically  represents  a  well-developed  thunder- 
storm in  progress.  The  falling  rain,  often  mixed  with  hail, 
cools  the  air  through  which  it  falls;  and  as  the  temperature 
gradient  was  already  closely  adiabatic  it  follows  that  the 
actual  temperatures  will  be  subpotential  from  the  surface  of 
the  earth  to  within  the  cloud,  or  throughout  and  a  little 
beyond  the  nonsaturated  or  evaporating  levels.  As  soon, 
then,  as  this  column  or  sheet  of  air  is  sufficiently  cooled  it 
flows  down  and  forward,  and  all  the  atmospheric  movements 
peculiar  to  the  thunderstorm  are  established,  substantially 
as  shown. 

"Referring  to  the  figure,  the  warm  ascending  air  is  in  the 
region  A;  the  cold  descending  air  at  D;  the  dust  cloud  (in 
dry  weather)  at  D' ,  the  squall  cloud  at  5;  the  storm  collar  at 
C;  the  thunder  heads  at  T;  the  hail  at  H;  the  primary  rain, 
due  to  initial  convection,  at  R,  and  the  secondary  rain  at  R' '. 
This  latter  phenomenon,  the  secondary  rain,  is  a  thing  of 
frequent  occurrence  and  often  is  due,  as  indicated  in  the 


ATMOSPHERIC  ELECTRICITY 


181 


figure,  to  the  coalescence  and  quiet  settling  of  drops  from  an 
abandoned  portion  of  the  cumulus  in  which  and  below  which 
winds  and  convection  are  no  longer  active. 

' '  Mammato-cumuli  rarely,  false  cirri  frequently,  and  cap- 
clouds  occasionally,  accompany  thunderstorms,  but  as  they 


JSa3BHi!i!!!!l 


D 


FIG.  68.     IDEAL  CROSS-SECTION  OF  A  TYPICAL  THUNDERSTORM 
A,  ascending  air;   D,  descending  air;    C,  storm  collar;   S,  roll  scud;   D',  wind  gust; 
H,  hail;  T,  thunder  heads;  R,  primary  rain;  R',  secondary  rain 

are  not  essential  to  a  thunderstorm  they  therefore  are  omitted 
from  the  above  schematic  illustration. 

"Before  the  onset  of  a  thunderstorm  there  usually,  if  not 
always,  is  a  distinct  fall  in  the  barometer.  At  times  this  fall 
is  extended  over  several  hours,  but  whether  the  period  be  long 
or  short  the  rate  of  fall  usually  is  greatest  at  the  near  approach 
of  the  storm.  Just  as  the  storm  breaks,  however,  the  pressure 
rises  very  rapidly,  almost  abruptly,  usually  from  1  to  3  kilobars 
(1  to  2  millimeters),  fluctuates  irregularly,  and  finally,  as  the 
storm  passes,  again  becomes  rather  steady  but  at  a  somewhat 
higher  pressure  than  prevailed  before  the  storm  began. 

"The  cause  of  these  pressure  changes  is,  doubtless,  rather 
complex.  The  decrease  in  the  absolute  humidity  and  the 
decrease  in  temperature  both  tend  to  increase  the  atmos- 
pheric pressure,  and,  presumably,  each  contributes  its  share. 
Both  these  effects,  however,  are  comparatively  permanent, 
and  while  they  may  be  mainly  responsible  for  the  increase 
of  pressure  that  persists  after  the  storm  has  gone  by,  they 
probably  are  not  the  chief  factors  in  the  production  of  the 
initial  and  quickly  produced  pressure  maximum.  Here  at 


182  THE  PRINCIPLES  OF  A&ROGRAPHY 

least  two  factors,  one  obvious,  the  other  inconspicuous,  are 
involved.  These  are  (a)  the  rapid  downrush  of  air,  and  (b) 
the  interference  to  horizontal  flow  caused  by  the  vertical 
circulation. 

;<The  downrush  of  air  clearly  produces  a  vertically  directed 
pressure  on  the  surface  of  the  earth,  in  the  same  manner 
that  a  horizontal  flow  produces  a  horizontally  directed  pres- 
sure against  the  side  of  a  house.  But  a  pressure  equal  to 
a  force  of  3  kilobars,  a  pressure  increase  frequently  reached 
in  a  thunderstorm,  would  mean  about  3  grams  per  square 
centimeter,  or  30  kilograms  per  square  meter,  and  require  a 
wind  velocity  of  roughly  50  kilometers  per  hour,  or  14  meters 
per  second.  Now  the  velocity  of  the  downrush  of  air  in  a 
thunderstorm  is  not  at  all  accurately  known,  but  while  at 
times  probably  very  considerable,  the  above  value  of  14  meters 
per  second  seems  to  be  excessive;  in  fact,  its  average  value  may 
not  be  even  half  so  great.  If  in  reality  it  is  not,  then,  since 
the  pressure  of  a  wind  varies  as  the  square  of  its  velocity,  it 
follows  that  less  than  one  fourth  of  the  actual  pressure  increase 
can  be  caused  in  this  way.  Hence  it  would  seem  that  there 
probably  is  at  least  one  other  pressure  factor,  and,  indeed, 
such  a  factor  obviously  exists  in  the  check  to  the  horizontal 
flow  caused  by  vertical  convection. 

"Mere  mingling  of  the  two  air  currents,  upper  and  lower, 
cannot  change  the  depth  of  the  atmosphere,  nor,  therefore,  the 
height  of  the  barometer.  But  in  the  case  of  atmospheric  con- 
vection we  have  something  more  than  the  simple  mingling 
of  two  air  currents,  and  the  linear  momentum  does  not,  in 
general,  remain  constant.  The  increased  surface  velocity 
following  convection,  a  phenomenon  very  marked  in  the  case 
of  a  thunderstorm,  causes  an  increased  frictional  drag  and 
therefore  a  greater  or  less  decrease  in  the  total  flow.  Sup- 
pose this  amounts  to  the  equivalent  of  reducing  the  velocity 
of  a  layer  of  air  only  25  meters  thick  from  V  to  v,  and  let 
V=5v.  That  is,  the  one  three-hundred-and-twentieth  part 
of  the  atmosphere  has  its  flow  reduced  to  one  fifth  its  former 
value.  This  would  reduce  the  total  flow  by  about  1  part 
in  400,  and  thereby  increase  the  barometric  reading  by  nearly 
3  kilobars. 


ATMOSPHERIC  ELECTRICITY  183 

"It  would  seem,  then,  that  the  friction  of  the  thunder- 
storm gust  on  the  surface  of  the  earth,  through  the  consequent 
decrease  in  the  total  linear  momentum  of  the  atmosphere  and, 
therefore,  its  total  flow,  must  be  an  important  contributing 
cause  of  the  rapid  and  marked  increase  of  the  barometric  pres- 
sure that  accompanies  the  onset  of  a  heavy  thunderstorm. 

"To  sum  up:  The  chief  factors  contributing  to  the  in- 
crease of  the  barometric  pressure  during  the  thunderstorm 
appear  to  be,  possibly,  in  the  order  of  their  magnitude,  (a)  de- 
crease of  horizontal  flow,  due  to  surface  friction;  (6)  vertical 
wind  pressure,  due  to  descending  air;  (c)  lower  temperature; 
(d)  decrease  in  absolute  humidity. 

"Before  the  onset  of  the  storm  the  temperature  commonly 
is  high,  but  it  begins  rapidly  to  fall  with  the  first  outward  gust 
and  soon  drops  often  as  much  as  from  five  to  ten 
degrees;  because,  as  already  explained,  this  gust      temperature 
is  a  portion  of   the  descending  air  cooled   by  the 
cold  rain  and  by  its  evaporation.     As  the  storm  passes  the 
temperature  generally  recovers  somewhat,  though  it  seldom 
regains  its  original  value. 

"Heavy  rain,  at  least  up  in  the  clouds,  and  therefore  much 
humidity,   and  a  temperature  contrast  sufficient  to  produce 
rapid   vertical    convection,    are   essential   to    the 
genesis   of   a   thunderstorm.      Hence   during   the  humidity 

early  forenoon  of  a  thunderstorm  day  both  the 
absolute  and  the  relative  humidity  are  likely  to  be  high. 
Just  before  the  storm,  however,  when  the  temperature  has 
greatly  increased,  though  the  absolute  humidity  still  is  high, 
the  relative  humidity  is  likely  to  be  rather  low.  On  the 
other  hand,  during  and  immediately  after  the  storm,  because 
chiefly  of  the  decrease  in  temperature,  the  absolute  humidity, 
is  comparatively  low  and  the  relative  humidity  high. 

"It  has  frequently  been  noted  that  the  rainfall  is  greatest 
after  heavy  claps  of  thunder,  a  fact  that  appears  to  have  given 
much  comfort  and  great  encouragement  to  those 
who  maintain  the  efficacy  of  mere  noise  to  in-  gashes 

duce    precipitation  —  to     jostle     cloud    particles 
together  into  raindrops.      The  correct  explanation,  however, 
of  this  phenomenon  seems  obvious:    The  violent  turmoil  and 


184  THE  PRINCIPLES  OF  A&ROGRAPHY 

spasmodic  movements  within  a  large  cumulus  or  thunderstorm 
cloud  cause  similar  irregularities  in  the  condensation  and 
resulting  number  of  raindrops  at  any  given  level.  These 
in  turn,  as  broken  by  the  air  currents,  give  local  excess  of 
electrification  and  of  electric  discharge  or  lightning  flash. 
We  have,  then,  starting  toward  the  earth  at  the  same  time 
and  from  practically  the  same  level,  mass,  sound,  and  light. 
The  light  travels  with  the  greatest  velocity,  about  300,000 
kilometers  per  second,  and,  therefore,  the  lightning  flash  is 
seen  before  the  thunder  is  heard.  The  velocity  of  sound 

is  only  about  330  meters  per  second.  But 
Velocity  of  .  J.  *'.11  .  ,  r 

thunder  the    rain   falls   much   slower   still   and  therefore 

an.d,  reaches  the  ground  after  the  thunder  is  heard, 

raindrops  T  v  : P     .  .  .  . 

In  reality  it  is  the  excessive  condensation  or  ram 

formation  up  in  the  cumulus  cloud  that  causes  the  vivid 
lightning  and  the  heavy  thunder.  According  only  to  the 
order  in  which  their  several  velocities  cause  them  to  reach 
the  surface  of  the  earth  it  might  appear,  and  has  often  been 
so  interpreted,  that  the  lightning,  first  perceived,  was  the 
cause  of  the  thunder,  which,  indeed,  it  is,  and  that  the 
heavy  thunder,  next  in  order,  was  the  cause  of  the  excessive 
rain. 

"The  velocity  of  the  thunderstorm  is  simply  the  velocity 
of  the  atmosphere  in  which  the  bulk  of  the  cumulus  cloud 
Rate  of  happens  to  be  located.  Hence,  as  the  wind  at 

movement  this  level  is  faster  by  night  than  by  day  and 
faster  over  the  ocean  than  over  land,  it  follows 
that  exactly  the .  same  relations  hold  for  the  thunderstorm, 
that  it  travels  faster  over  water  than  over  land  and  faster  by 
night  than  by  day.  The  actual  velocity  of  the  thunderstorm, 
of  course,  varies  greatly,  but  its  average  velocity  in  Europe 
is  30  to  50  kilometers  per  hour;  in  the  United  States,  50  to  65 
kilometers  per  hour. 

"Hail,  consisting  of  lumps  of  roughly  concentric  layers  of 
compact  snow  and  solid  ice,  is  a  conspicuous  and  well-known 
Hail  phenomenon  that  occurs  during  the  early  portion 

of  most  severe  thunderstorms.  But  in  what  por- 
tion of  the  cloud  it  is  formed  and  by  what  process  the  layers 
of  ice  and  snow  are  built  up  are  facts  that,  far  from  being 


ATMOSPHERIC  ELECTRICITY  185 

obvious,  become  clear  only  when  the  mechanism  of  the  storm 
itself  is  understood. 

"As  before,  let  the  surface  temperature  be  303° A.  and  the 
absolute  humidity  40  per  cent,  or  the  dew  point  288° A. 
Under  these  conditions  saturation  will  obtain,  and,  there- 
fore, cloud  formation  will  begin  when  the  surface  air  has 
risen  to  an  elevation  of  1.5  kilometers.  Immediately  above 
this  level  the  latent  heat  of  condensation  reduces  the  rate  of 
temperature  decrease  with  elevation  to  about  half  its  former 
value,  nor  does  this  rate  rapidly  increase  with  further  gain 
of  height.  Hence,  usually,  for  the  above  assumptions  cor- 
respond in  general  to  average  thunderstorm  conditions,  it  is 
only  beyond  the  4-kilometer  level  that  freezing  temperatures 
are  reached.  It  is  therefore  only  in  the  upper  portions  of 
cumulus  clouds,  the  portions  that  clearly  must  consist  of 
snow  particles  and  undercooled  fog  or  cloud  droplets,  that 
hail  can  either  originate  or  greatly  grow. 

"But  what,  then,  is  the  process  by  which  the  nucleus  of 
the  hailstone  is  formed,  and  its  layer  upon  layer  of  snow  and 
ice  built  up?  Obviously  such  drops  of  rain  as  the  strong 
updraft  within  the  cloud  may  blow  into  the  region  of  freezing 
temperatures  will  quickly  congeal  and  also  gather  coatings 
of  snow  and  frost.  After  a  time  each  incipient  hailstone  will 
get  into  a  weaker  updraft,  for  this  is  always  irregular  and 
puffy,  or  else  will  tumble  to  the  edge  of  the  ascending  column. 
In  either  case  it  will  then  fall  back  into  the  region  of  liquid 
drops,  where  it  will  gather  a  coating  of  water,  a  portion  of 
which  will  at  once  be  frozen  by  the  low  temperature  of  the 
kernel.  But  again  it  meets  an  upward  gust,  or  falls  back 
where  the  ascending  draft  is  stronger,  and  again  the  cyclic 
journey  from  realm  of  rain  to  region  of  snow  is  begun;  and 
each  time — there  may  be  several — the  journey  is  com- 
pleted a  new  layer  of  ice  and  a  fresh  layer  of  snow  are  added. 
In  general,  the  size  of  the  hailstones  will  be  roughly  propor- 
tional to  the  strength  of  the  convection  current,  but  since 
their  weights  vary  approximately  (they  are  not  homogeneous) 
as  the  cube  of  their  diameters  while  the  supporting  force  of 
the  upward  air  current  varies,  also  approximately,  as  only  the 
square  of  their  diameters,  it  follows  that  a  limiting  size  is 


186  THE  PRINCIPLES  OF  A&ROGRAPHY 

quickly  reached.  It  is  also  evident,  from  the  fact  that  a 
strong  convection  current  is  essential  to  the  formation  of 
hail,  that  it  can  occur  only  where  this  convection  exists; 
that  is,  in  the  front  portion  of  a  heavy  to  violent  thunder- 
storm. 

"Some  meteorologists  hold  that  the  roll  scud  between  the 
ascending  warm  and  descending  cold  air  is  the  seat  of  hail 
Hail  not  formation,  but  this  is  a  mistaken  assumption. 

due  to  Centrifugal  force  would  throw  a  solid  object,  like 

roll  scud  a  hailstone,   out   of  this  roll  probably  before   a 

single  turn  had  been  completed.  Besides,  and  this  objection 
is,  perhaps,  more  obviously  fatal  than  the  one  just  given, 
the  temperature  of  the  roll  scud,  because  of  its  position,  the 
lowest  of  the  whole  storm  cloud,  clearly  must  be  many  degrees 

above  the  freezing 
point.  Indeed, 
temperatures  low 
enough  for  the 
formation  of  hail 
cannot  obtain  at 
levels  much  less 
than  three  times 
Walter  that  of  the  scud. 

FIG.  69.     THE  GROWTH  OF  AN  ELECTRIC  SPARK  Therefore,      it      is 

DISCHARGE  t          -,         •          , -. 

clearly    in    the 

higher  levels  of  the  cumulus  and  not  in  the  low  scud  that 
hail  must  have  its  genesis  and  make  its  growth." 

49.  Lightning.  Dr.  Walter  of  Hamburg  has  obtained,  by 
means  of  a  rotating  camera,  proof  that  lightning  flashes  are 

not,     as    heretofore    generally    thought    to    be, 
Lightning  '  fe  J  °  1 

flashes  not        oscillatory  discharges,  but  mainly  unidirectional. 

dischar°es  A  lightning  flash  begins  with  a  preliminary 
branching  spark  followed  within  a  brief  interval, 
say  .01  second,  by  another  rupture  or  discharge  somewhat 
longer,  until  finally  a  path  is  built  up  for  a  main-line  dis- 
charge, which  again  is  intermittent.  According  to  this  view,  a 
discharge  is  something  like  a  tearing  or  ripping,  but,  of  course, 
done  in  a  very  brief  interval  of  time.  Many  photographs, 
however,  serve  to  indicate  this  progressive  character  of 


ATMOSPHERIC  ELECTRICITY 


187 


Walter 

FIG.  70.     STREAK  LIGHTNING,  STATIONARY 

CAMERA 
Companion  to  Fig    71 


the   main   line   of   break- 
down of  the  dielectric,  or 
air,    (Figs.  69,  70,  and  71.) 
Dr.    Wilhelm    Schmidt 
has  pointed  out  that,  be- 
cause of  the  great  accu- 
mulation  of  energy  in  a 
very    small    space,    there 
must  be   a   strong  repul- 
sion of  electrified  air  par- 
ticles of  like   sign  and  a 
sudden  increase  of   pres- 
sure in  the   path   of   the 
discharge.     This     may 
amount    to    perhaps    100 
atmospheres.     A   wave, 
therefore,  of  intense  energy  spreads  out  in  all  directions  —  an 
explosion  wave.     There  is  a  strong  push  or  condensation,  and 
rebound  or  rarefaction  succeeded  by  waves  of  less 
intensity.     For  studying  the  wave  motion  during     discharge  an 
thunderstorms,  Schmidt  used  two  forms  of  appa- 
ratus,  one  for  analysis  of  regular  sound  waves  and 
the  other  for  the  longer  pressure  waves.     The  records  showed 

that  regular  trains  of  waves 
of  uniform  length  prac- 
tically never  occurred  and 
hence  that  thunder  had 
no  proper  "tone,"  but  was 
merely  a  "noise"  similar 
in  character  to  the  rattling 
of  window  panes.  Irregu- 
larity was  greatest  during 
the  heaviest  thunder,  while 
near  the  end  of  the  peal 
a  certain  regularity  was 
noticed.  A  statistical 
waiter  analysis  of  the  frequency 

FIG.  71.     STREAK  LIGHTNING  (SEQUENT          w{th  which  waves  of  various 
DISCHARGE),  ROTATING  CAMERA 

Companion  to  Fig.  70  lengths  occurred  showed  a 


188  THE  PRINCIPLES  OF  A&ROGRAPHY 

preponderance  of  those  lasting  .025  second  or  more,  and 
again  of  those  lasting  between  .0083  and  .013  second,  that  is, 
vibrations  of  such  length  that  if  they  had  occurred  in  uniform 
trains  they  would  have  produced  the  tone  E  or  lower,  or 
again  tones  between  D  and  A.  Shorter  waves  most  common 
in  music  were  rarer. 

The  fluctuations  in  air  density  occurring  with  these  waves 
were  larger  than  in  ordinary  sound  waves.  Although  the 
lightning  was  never  close  at  hand,  the  interval  between 
flash  and  beginning  of  thunder  being  about  5  seconds,  the 
pressure  fluctuations  were,  as  a  rule,  more  than  1.3  kilobars 
(1  millimeter).  ''Hence,"  says  Schmidt,  "the  greater  part 

of  the  total  energy  of  thunder  is  represented  by 
Only  a  small  -,-,1  1  ^1 

part  of  a  clap     long,  inaudible  waves,  and,  strange  as  the  state- 

ment  maY  sound,  we  may  say  that  one  really 
hears  only  the  smallest  part  of  a  clap  of  thunder. 
Most  of  the  phenomenon  either  escapes  our  senses  altogether 
or  is  perceptible  only  through  the  vibration  of  objects  around 
us,  as  the  rattling  of  window  panes.  In  the  immediate 
vicinity  of  the  discharge  the  pressure  fluctuations  must  be 
very  violent,  and  much  of  the  purely  mechanical  injury 
wrought  by  lightning  may  be  due  to  them." 

The  number  of  violent  waves  is  small,  occurring  generally 
in  irregular  series  of  three  or  four.  In  the  heaviest  thunder 
claps  there  is  usually  but  one  violent  wave, —  at  the  begin- 
"Sh  k  "  n^n&-  Such  a  wave,  called  a  "shock  wave," 
travels  in  all  directions  from  the  path  of  the 
electrical  discharge.  The  prolongation  of  the  sound  is  due 
to  the  fact  that  the  discharge  is  perhaps  intermittent  and 
may  set  up  several  initial  waves;  also  because  of  reflection, 
not  so  much  from  clouds  and  sheets  of  falling  rain  as  from 
the  "interfaces"  between  atmospheric  strata  of  different 
temperatures — and  especially  by  the  action  of  the  wind. 
The  original  sharp  report  is  transformed  into  a  "roll." 
Irregular  short  waves,  which  give  the  rattling  noise  of  near-by 
thunder,  are  gradually  lost  in  the  more  regular  waves  so  that 
in  distant  thunder  the  sound  may  have  a  definite  pitch.  The 
energy  of  thunder  as  shown  by  these  records  amounted  in  a 
maximum  case  to  22,000  kilogrammeters,  and  was  therefore 


ATMOSPHERIC  ELECTRICITY  189 

very  great  compared  with  that  of  ordinary  sounds.  In  this 
case  the  thunder  lasted  13  seconds,  and  it  would  require  more 
than  200,000,000  buglers,  blowing  for  the  same  Tremendous 
length  of  time,  to  produce  an  equivalent  amount  energy  of 
of  energy.  Nevertheless,  this  quantity  is  insig- 
nificant compared  with  that  of  a  flash  of  lightning,  for  which 
we  may  assume,  and  not  in  extreme  cases,  something  like 
10,000,000,000  kilogrammeters.  In  fact,  only  a  small  part 
of  the  energy  of  lightning  is  transformed  into  pressure  waves 
and  sound,  most  of  it  assuming  other  forms,  such  as  those 
of  heat  or  light. 

With  regard  to  the  explanation  of  the  "rolling"  of  thunder 
as  due  to  the  fact  that  the  sound  reaches  the  observer  first 
from  the  nearest  part  of  the  path  and  later  from 
the  more  distant  parts,  and  that  the  duration  of    of  thunder18 
thunder  depends  upon  these  intervals, —  Schmidt 
points  out  that  in  the  case  of  a  uniform  impulse  along  a  path 
free  from  sharp  angles  there  would  be  only  a  single  wave, 
spreading  in  all  directions,  and  that  the  observer  would  there- 
fore perceive  only  a  single  brief  sound,  and  that  the  time  of 
occurrence  would  depend  upon  the  nearest  part  of  the  lightning 
path.     Bends   in   the   lightning  path  will  account  only  for  a 
limited  number  of  "claps,"  and  not  for  the  " roll "  of  thunder. 

It  has  been  suggested  by  Miessner  that  there  may  be 
electrolytic  and  thermal  decomposition  of  the  water  vapor 
and  subsequent  explosion  by  the  lightning.  Further  experi- 
mentation is  necessary. 

L.  A.  De  Blois  has  made  special  investigations  on  the 
character  of  the  lightning  discharge.  Of  fourteen  electrical 
storms  in  the  summer  of  1913,  six  were  of  a  character  suitable 
for  his  observations,  which  were  made  primarily  to  determine 
suitable  protection  for  buildings  containing  explosives.1  He 
found  that  after  some  experience  with  the  sound  produced  in 
wireless  receivers  by  waves  propagated  by  lightning  discharges, 
he  could  predict  the  occurrence  of  the  storm  by  eight  or  ten 
hours.  His  instruments  were  a  wireless  receiving  outfit  with 

1  De  Blois'  results  are  published  in  a  paper  presented  before  the  294th 
meeting  of  the  American  Institute  of  Electrical  Engineers,  April  24-25,  1914. 
A  criticism  by  C.  F.  Marvin  of  the  deductions  drawn  from  these  experiments 
can  be  found  in  the  Monthly  Weather  Review  for  Aug.,  1914. 


190 


THE  PRINCIPLES  OF  A&ROGRAPHY 


various  detectors, 
an  indie  a  t  in  g 
ceraunoscope,  a 
static  voltmeter, 
and  an  oscillo- 
graph, which,  when 
in  circuit,  dis- 
charged at  a  fre- 
quency of  about 
400,000  oscillations 
per  second.  The 
natural  period  of 
the  oscillograph  it- 
self was  only  about 
5,000.  DeBlois 
gives  three  char- 
acteristic lightning 
oscillograms.  The 
first  was  a  typical 
example  of  a  single 
steep-front  dis- 
charge from  clouds 
to  earth.  The 
maximum  value  of 
the  induced  current 
was  0.5  ampere, 
and  the  peak 
proper  endured 
0.0008  second. 
The  second  was  a 
typical  steep-front  wave  with  five  main  and  three  supplement- 
ary peaks.  The  third  was  a  wave  of  totally  different  charac- 
ter. The  direction  of  flow  is  from  the  earth  upward,  but  with 
a  gradual  rise,  with  indications  of  oscillations  to  a  maximum 
of  0.18  ampere,  which  is  reached  in  0.0046  second.  De  Blois 
sums  up  his  photographic  records  of  50  discharges  as  follows: 

Total  number  of  strokes  positive  (from  clouds  to  earth),  43; 
Total  number  of  strokes  positive  (from  earth  to  clouds),  7; 
Single-peak  discharges,  38,  average  duration  0.00065  second; 
Multiple-peak  discharges,  12. 


McAdie 

FIG.  72.     LIGHTNING  DISCHARGE  THROUGH  CLOUDS 


ATMOSPHERIC  ELECTRICITY 


191 


De  Blois  states  that  in  no  record 
was  there  any  indication  of  regular 
periodic  high-frequency  oscillations  in 
the  induced  current.  The  peaks  may 
represent  simply  the  result  of  the 
''progressive  breakdown"  of  the  at- 
mosphere, defined  by  Steinmetz;  that 
is,  a  discharge  from  point  to  point. 
Steinmetz  has  likened  the  discharge  of 
a  cloud  to  a  landslide  which  sets  off  a 
series  of  landslides.  He  asks  us  to 
imagine  a  relief  map  of  wet  sand,  the 
hills  representing  the  dense  portion  of 
the  cloud  or  the  places  of  high  poten- 
tial, and  the  valleys  the  places  of  light, 
or  low,  potential.  Where  the  declivity 
is  very  steep  a  slide  occurs  which 
causes  another  slide  and  so  on  until 
the  hills  are  leveled  and  the  valleys 
filled;  or,  in  other  words,  until  the 
electric  potential  is  equalized.  What- 
ever may  be  the  true  explanation,  it 
would  appear  that  the  most  dangerous 
discharges  are  those  with  almost  in- 
stant rise  to  maximum  value. 

Humphreys  states  that  "curious 
luminous  balls  or  masses,  of  which 
C.  De  Jans1  probably  has  given  the 
fullest  account,  have  time  and  again 
been  reported  among  the  phenomena 
observed  during  a  thunderstorm.  Most 
of  them  appear  to  last  only  a  second 
or  two  and  to  have  been  seen  at  close 
range,  some  even  passing  through  a 
house,  but  they  have  also  appeared  to 
fall,  as  would  a  stone  or  like  a  meteor, 
from  the  storm  cloud,  and  along  the 
approximate  path  of  both  previous 

1  del  et  terre,  Bruxelles,  Vol.  XXXI  (1910) ,  p.  499. 


McAdie 

FIG.   73.     LIGHTNING 

FLASH 
Note  the  dark  flashes. 


192 


THE  PRINCIPLES  OF  AEROGRAPHY 


and  subsequent 
lightning  flashes. 
Others  appear  to 
start  from  a  cloud 
and  then  quickly 
return,  and  so  on 
through  an  endless 
variety  of  places 
and  conditions. 

"D  oubtless 
many  reported 
cases  of  ball  light- 
ning are  entirely 
spurious,  being 
either  fixed  or 
wandering  brush 
discharges  or  else 
nothing  other  than 
optical  illusions, 
due  in  most  cases 
probably  to  per- 
sistence of  vision. 
But  here,  too,  as 
in  the  case  of 
rocket  lightning, 
the  amount  and 
excellence  of  obser- 
vational evidence 
forbid  the  assump- 
tion that  all  such 
phenomena  are 
merely  subjective. 
Possibly  in  some 
instances,  especial- 
ly those  in  which 
it  is  seen  to  fall 
from  the  clouds,  ball  lightning  may  be  only  extreme  cases  of 
rocket  lightning,  cases  in  which  the  discharge  for  a  time  just 
sustains  itself.  A  closely  similar  idea  has  been  developed  in 


FIG.  74. 


Photograph  by  A.  Steadworthy,  Dom.  Astr.  Obs.  Ottawa, 
in  Jour,  of  the  Royal  Astr.  Soc.  of  Canada,  1912 

PHOTOGRAPH  OF  LIGHTNING  TAKEN  IN 
DAYLIGHT,  JULY  10,   1912 


ATMOSPHERIC  ELECTRICITY  193 

detail  by  Toepler.  The  ball  may  either  disappear  wholly  and 
noiselessly,  as  often  reported,  or  it  could  perhaps  suddenly 
gain  in  strength  and  instantly  disappear,  as  sometimes  ob- 
served, with  a  sharp,  abrupt  clap  of  thunder. 

"To  say  that  all  genuine  cases  of  ball  lightning,  those  that 
are  not  mere  optical  illusions,  are  stalled  thunderbolts  cer- 
tainly may  sound  very  strange.     But  that,  indeed,        fi 
is  just  what  they  are  according  to  the  above  specu-        lightning 
lation — a  speculation  that  recognizes  no  difference 
in  kind  between  streak,  rocket,  and  ball  lightning, 
only  differences  in  the  amounts  of  ionization,   quantities  ol 
available  electricity,  and  steepness  of  potential  gradients. 

' '  When  a  distant  thundercloud  is  observed  at  night  one  is 
quite  certain  to  see  in  it  beautiful  illuminations,  looking  like 
great  sheets  of  flame,  that  often  flicker  and  glow  Sheet 

in  exactly  the  same  manner  as  does  streak  light-  lightning 

ning  for  well-nigh  a  whole  second.  In  the  daytime  and  in  full 
sunlight  the  phenomenon  when  seen  at  all  appears  like  a  sudden 
sheen  that  travels  and  spreads  here  and  there  over  the  surface 
of  the  cloud.  Certainly  in  most  cases — so  far  as  definitely 
known  in  all  cases — this  is  only  reflection  from  the  body  of 
the  cloud  of  streak  lightning  in  other  and  invisible  portions. 
Conceivably  a  brush  or  coronal  discharge  may  take  place 
from  the  upper  surface  of  a  thunderstorm  cloud,  Glow 
but  one  would  expect  this  to  be  either  a  faint  con-  discharge 
tinuous  glow,  or  else  a  momentary  flash  coincident  with  a  dis- 
charge from  the  lower  portion  of  the  cloud  to  earth  or  to  some 
other  cloud.  But,  as  already  stated,  only  reflection  is  defi- 
nitely known  to  be  the  cause  of  sheet  lightning.  Coronal 
effects  seem  occasionally  possible,  but  that  they  are  ever  the 
cause  of  the  phenomenon  in  question  has  never  clearly  been 
established  and  appears  very  doubtful.  It  has  often  £>een 
asserted,  too,  that  there  is  a  radical  difference  between  the 
spectra  of  streak  and  sheet  lightning,  but  even  this  does  not 
appear  ever  to  have  been  photographically  proved." 

50.  Other  forms  of  discharge.  These  are  beaded  lightning, 
or  apparently  discontinuous  streaks;  return  strokes;  dark 
flashes  (Fig.  73),  which  are  believed  to  be  photographic  effects; 
heat  or  distant  discharges ;  St.  Elmo's  fire;  and  rocket  lightning. 

14 


194 


'THE  "PRINCIPLES  OF  XEROGRAPHY 


FIG.  75..    SPECTRUM  OF 
LIGHTNING 


"The  spectrum  of  a  lightning  flash  and  that  of  an  ordinary 
electric  spark  in  air  are  practically  identical.     This  is  well 

shown  by  Fig.  75,  copied  from  an 
article  on  the  spectrum  of  lightning 
by  Fox,1  in  which  the  upper  or  wavy 
portion  is  due  to  the  lightning  and 
the  lower  or  straight  portion  to  a 
laboratory  spark  in  air." 

A.  Steadworthy  gives  the  follow- 
ing description2  of  how  he  secured 
a  spectrum  of  lightning: 

"A  favorable  opportunity  offered 
on  the  night  of  July  10,  1912,  when 
a  storm  began  to  develop  at  6  P.M. 
at  Ottawa  in  the  west.  Low-lying 
cumulus  clouds  slowly  spread  toward 
the  north  and  south  where  distant 
lightning  first  manifested  itself.  The 
clouds,  becoming  denser  and  denser, 
approached  Ottawa  and  slowly  rose  above  the  horizon.  By 
7  P.M.  the  first  rumblings  of  thunder  began,  and  by  9:30  P.M. 
the  pyrotechnic  display  in  the  west  became  so  vivid  that  I 
brought  out  the  two  cameras  which  I  had  in  readiness  at  my 
house  overlooking  McDonald  Park,  giving  me  an  outlook  from 
the  west  through  the  north  to  the  east.  One  camera  was 
fit-ted  with  stereoscopic  lenses,  the  other  was  an  8-inch  by 
10-inch  camera  with  2-inch  lens,  16-inch  focus.  In  front  of 
it  was  fitted  in  a  box  one  of  our  60-degree  dense  flint-glass 
objective  prisms.  The  box  could  be  rotated  on  the  collar 
of  the  lens,  thereby  enabling  spectra  to  be  obtained  over  a 
range  of  180  degrees  without  moving  the  camera.  It  must 
be  observed  here  that  the  size  of  the  small  box  and  the  prism 
not  physically  covering  the  lens  made  it  possible  for  a  flash 
coming  from  a  certain  direction  to  get  through  the  lens  and 
on  to  the  plate  without  passing  through  the  prism.  And 
this  is  exactly  what  happened,  as  seen  in  Fig.  76,  which  shows 
the  spectrum  secured  with  the  superimposed  other  flash. 

1  Astrophysical  Journal,  Chicago,  Vol.  XVIII  (1903),  p.  294 

2  Jour,  of  the  Royal  Astr.  Soc.,  Canada,  Nov.-Dec.,  1912. 


Courtesy  of  Dr.  C .  A.  Chant 
From  Jour,  of  the  Royal  Astr.  Soc.  of  Canada,  Vol.  8  (1914),  p.  346 

FIG.  76.     SPECTRUM  OF  LIGHTNING 

This  photograph  was  taken  at  Ottawa  by  A.  Steadworthy,  July  10,  1912.     The 
straight  streaks  at  the  bottom  are  spectra  of  street  lamps. 


195 


196 


THE  PRINCIPLES  OF  ARROGRAPHV 


as 


'I  made  four  exposures  on  Cramer  isochromatic  plates,  for, 
I  thought,  four  spectra.     One  of  the  plates  turned  out  a 


From  Jour,  of  the  Royal  Astr.  Soc.  of  Canada,  Vol.  8  (1914),  p.  345 

FIG.  77.     SPECTRUM  OF  LIGHTNING 

This  is  a  portion  of  Fig.  76,  with  wave-lengths  added.     It  is  magnified  to  five 
times  the  original. 

blank,  one  showed  a  very  weak  spectrum,  one  showed  the 
fine  spectrum  on  the  above  plate,  while  the  last  showed 
remarkable  dotted  curved  lines,  one  at  the  top  of  the  plate 
and  several  parallel  ones  below.  I  utterly  fail  to  divine 
their  origin.  The  exposure  and  the  position  of  the  camera 
were  identical  in  the  two  cases.  None  of  the  other  three 
plates  shows  this  phenomenon." 

LINES  IN  SPECTRUM  OF  LIGHTNING l 


Wave 
length 

Character 

Fox's  wave 
lengthf 

5683  .  0* 
5618.3 

Strong,  well-defined  line 
Broad 

5683 
5600 

r.  edge 
max. 

5556.3 
5423.2 

Broad  and  strong 
Violet  edge  of  gradually  fading  band 

5345.5 
5180.9 

Faint,  broad  band 
Faint,  broad  line 

5306 
5175 

v.  edge 

5003.7* 
4928.5 

Good,  strong  band 
Faint  line 

5003.7 

4852.4 
4800.7 

Broad,  fairly  strong  band 
Faint  line 

4842 
4786 

4688.5 

Fairly  good  line 
Faint  indistinct  lines  between  4688  .  5  and 

4660 

r.  edge 

4648.9 

4648.9 

Faint  line 

4635.6 

Strong,  clear  line 
More  faint  lines 

4630.7 

4605.3 

Fairly  strong  broad  lines 

4603 

v.  edge 

1  By  A.  Steadworthy  and  J.  B.  Cannon,   Dominion  Observatory,   Ottawa, 
Canada,  Sept.,  1914. 


ATMOSPHERIC  ELECTRICITY 


197 


LINES  IN  SPECTRUM  OF  LIGHTNING — Continued 


Wave 
length 

Character 

Fox's  wave 
lengthf 

4542.8 

Faint  line 

4535 

4522.8 

Fairly  strong  line 

14483.8 

A  pair  of  rather  weak  lines,  v.  one  the 

(4468.6 

stronger 

(4442.5 

Strong,  clear  line 

) 

4427.8 

Line  fainter  and  broader 

'4439 

(4413.4 

Very  faint  line 

1 

(4367.9 

Strong  line 

1 

U354.3 

Line  less  strong 

>4359 

(4340.9 

Fainter,  broad  line 

1 
i 

4308.1 

Faint,  broad  band 

4270.2 

Very  faint 

^ 

4264.1 

Very  faint 

4251.9 

Faint  line 

4239.8 

Strong  line 

4238 

4217.7 

Strong,  clear  line 

4216.1 

Faint  line 

4190.1 
4170.4 

Very  faint  line 
Fairly  strong  line 

J4183 

J4151.2 

Good,  strong  line 

4154 

14143.1 

Good  line,  not  so  strong  as  4151 

4127.0 

Very  faint  line 

}  4106.1 

Very  strong  line,  forms  a  close  strong  pair 

>4.in^ 

(4095.8 

with  4095.  8 

[  T:lUc) 

4070.6 

Rather  faint 

4077 

4041.3 

Good  strong  line 

4041.5 

4036.5 

Faint  line 

4024.7 

Faint  line 

4015.3 

Faint  line 

3997.0* 

Good,  strong  line 

3997 

(3959.2 
13952.7 

Faint  pair 

3943 

(3925.2 
1  3919.0 

Faint  pair 

3915 

3898.7 

Faint  line 

3890 

3888.7 

Very  faint 

3838 

*Lines  marked  thus  were  used  as  standards  in  the  compilation  of  wave  lengths. 
]Astrophysical  Journal,  Chicago,  Vol.  XVIII  (1903),  p.  294. 


m  THE  PRINCIPLES  OF  AEROGRAPHY 

51.  Destructive  effects  of  lightning.  One  of  the  most 
striking  effects  of  lightning  is  the  shattering  of  objects  struck. 
The  discharge  apparently  prefers  the  surface  and  the  effects 
are  largely  superficial.  Thus  the  clothes  are  sometimes  torn 
from  the  body,  splinters  of  flagpoles  are  thrown  great  distances, 
and  it  has  happened  that  where  three  men  have  been  standing 
under  a  tree,  the  one  in  the  center  of  the  group  was  only  injured, 
while  the  man  on  his  right  and  the  man  on  his  left  were  killed. 
Such  an  occurrence  was  reported  at  Lowell,  Mass.,  on  July  8, 
1916.  Humphreys  explains  the  explosive  effect  as  follows: 

"The  excessive  and  abrupt  heating  caused  by  the  lightning 

current  explosively  and  greatly  expands  the  column  of  air 

through    which    it    passes.     It    even    explosively 

effects^6          vaporizes  such  volatile  objects  as  it  may  hit  that 

have  not  sufficient  conductivity  to  carry  it  off. 

Hence,  chimneys  are  shattered,  shingles  torn  off,  trees  stripped 

of  their  bark  or  utterly    slivered   and   demolished,  kite  and 

other  wire  fused  or  volatilized,  holes  melted  through  steeple 

bells  and  other  large  pieces  of  metal,  and  a  thousand  other 

seeming  freaks  and  vagaries  wrought. 

"Many  of  the  effects  of  lightning  appear  at  first  difficult 
to  explain,  but,  except  the  physiological  and,  probably,  some 
of  the  chemical,  all  depend  upon  the  sudden  and  intense 
heating  along  its  path. 

"As  is  well  known,  oxides  of  nitrogen  and  even  what  might 
be  termed  the  oxide  of  oxygen,  or  ozone,  are  produced  along 
the  path  of  an  electric  spark  in  the  laboratory. 
effec?sCal          Therefore,  one  might  expect  an  abundant  forma- 
tion, during  a  thunderstorm,  of  these  same  com- 
pounds.    And  this  is  exactly  what  does  occur,  as  observation 
abundantly  shows.     It  seems  probable,  too,  that  some  ammo- 
nia must  also  be  formed  in   this   way,   the  hydrogen   being 
supplied  by  the  decomposition  of  raindrops  and  water  vapor. 
"In  the  presence  of  water  or  water  vapor  these  several 
compounds    undergo    important    changes    or    combinations. 
The  nitrogen  peroxide  (most  stable  of  the  oxides  of  nitrogen) 
combines  with  water  to  produce  both  nitric  and  nitrous  acids; 
the  ozone  with  water  gives  hydrogen  peroxide  and  sets  oxygen 
free;   and  the   ammonia  in   the  main   merely   dissolves,   but 


ATMOSPHERIC  ELECTRICITY  199 

probably  also  to  some  extent  forms  caustic  ammonia  and 
hydrogen. 

"The  ammonia  and  also  both  the  acids  through  the  pro- 
duction of  soluble  salts  are  valuable  fertilizers.  Hence, 
wherever  the  thunderstorm  is  frequent  and  severe,  especially, 
therefore,  within  the  tropics,  the  chemical  actions  of  the 
lightning  may  materially  add,  as  has  recently  been  shown,1 
to  the  fertility  of  the  soil  and  promote  the  growth  of  crops. 

"The  only  reason  for  mentioning  normal  atmospheric  elec- 
tricity in  connection  with  thunderstorms  is  to  emphasize  the 
fact  that,  contrary  to  what  many  suppose  to  be      Normal 
the  case,  there  is  but  little  relation,  in  the  sense      atmospheric 
of  cause  and  effect,  between  these  two  phenomena.      electncity 
Thus  while  the  difference  in  electrical  potential  between  the 
surface  of  the  earth  and  a  point  at   constant  elevation  is 
roughly  the  same  at  all  parts  of  the  world,  the  number  and 
intensity  of  thunderstorms  vary  greatly  from  place  to  place. 
Further,  while  the  potential  gradient  at  any  given    p0tential 
place  is  greatest  in  winter,  the  number  of  thunder-    gradient  and 
storms  is  most  frequent  in   summer,   and  while    thunderstorm 
the  gradient,  in  the  lower  layer  of  the  atmosphere,  at  many 
places,  usually  is  greatest  from  8  to  10  o'clock,  both  morning 
and  evening,    and  least  at  2  to  3  o'clock  P.M.  and  3  to  4 
o'clock    A.M.,    no    closely    analogous   relations   hold    for   the 
thunderstorm." 

However,  it  should  be  noted  that  there  are  marked  fluctua- 
tions in  the  potential  values  during  thunderstorms  and  snow 
storms.  Characteristic  curves  are  those  shown  in  Figs.  78 
and  79  obtained  by  the  author  in  experiments  made  at  the  top 
of  the  Washington  Monument  and  at  Blue  Hill  Observatory. 

"Probably  the  most  interesting  conclusion  in  regard  to 
normal  atmospheric  electricity  so  far  drawn  from  observa- 
tion and  experiment  is  this:  that  the  earth  everywhere,  land 
and  water  and  from  pole  to  pole,  is  a  negatively  charged 
sphere  of  practically  constant  surface  density,  surrounded  by 
an  atmosphere  of  such  conductivity  that  it  is  constantly 
carrying  away  a  current  that  amounts  on  the  whole  to  about 
1,000  amperes. 

i  Capus  in  Ann.  de  Geographic,  Paris,  Vol.  XXIII  (1914),  p.  109, 


200 


THE  PRINCIPLES  OF  A&ROGRAPHY 


FIG.  78.     VOLTAGE  OF  AIR  DURING  A  THUNDERSTORM  AT  THE  TOP 
OF  THE  WASHINGTON  MONUMENT 

"Where  the  supply  of  negative  electricity  comes  from  that 
keeps  the  surface  of  the  earth  on  the  whole  negatively  charged 
in  spite  of  this  steady  great  loss,  or  how  the  outgoing  current 
is  compensated,  no  one  knows.  Rain  does  not  help  matters, 
for,  as  we  have  seen,  rain  is  prevailingly  positive,  and  what  is 
needed,  to  compensate  the  loss,  is  negative  electricity  and  a  great 
deal  of  it.  Neither,  so  far  as  known,  is  negative  electricity 
supplied  by  means  of  the  lightning,  for,  in  the  great  majority 
of  cases,  this,  too,  is  positive  that  descends  from  cloud  to 
earth.  And  so  the  puzzle  remains.  As  Simpson1  puts  it: 
'  'A  flow  of  negative  electricity  takes  place  from  the  surface 
of  the  whole  globe  into  the  atmosphere  above  it,  and  this  neces- 
sitates a  return  current  of  more  than  1,000  amperes;  yet  not  the 
slightest  indication  of  any  such  current  has  so  far  been  found, 
and  no  satisfactory  explanation  for  its  absence  has  been  given." 

Detailed  descriptions  of  methods  of  obtaining  curves  of 
potential  may  be  found  in  the  Monthly  Weather  Review,  July, 

1  Nature,  London,  Vol.  XC  (1912),  p.  411. 


ATMOSPHERIC  ELECTRICITY 


201 


1891,  p.  171;  also  in  Third  Memoir  National  Academy  of  Sci- 
ences, Vol.  V,  by  T.  C.  Mendenhall;  also  in  Luftelectrizitdt, 
by  Karl  Kaehler,  Berlin,  1913. 

In  the  experiments  on  atmospheric  electricity  made  by 
Hewlett,  Kidson,  and  Johnston  of  the  Department  of  Ter- 
restrial Magnetism  of  the  Carnegie  Institution,  the  most 
striking  result  appears  to  be  that,  while  the  conductivity 
over  the  ocean  is,  on  the  average,  at  least  as  great  as  that 
over  land,  the  radioactive  content  is  much  smaller.  The 
mean  value  of  the  potential  gradient  near  the  surface  of  the 
water  is  approximately  120  volts  per  meter. 

There  appears  to  be  a  relation  between  conductivity  and 
temperature  and  pressure,  but  no  relation  to  absolute 
humidity.  An  elaborate  discussion  of  the  observations  made 
on  the  second  cruise  of  the  Carnegie  is  given  by  Hewlett 
in  Terrestrial  Magnetism,  September,  1914,  p.  127.  Specific 
conductivity,  potential  gradient,  and  radioactivity  observa- 
tions were  continued  for  more  than  two  years  in  the  Pacific, 
Atlantic,  and  Indian  oceans.  The  potential  gradient  over 
the  ocean  is  of  the  same  order  of  magnitude  as  over  the  land. 


4O  5O  6O  1O  2O  3O  4O 

FIG.  79.     VOLTAGE  OF  AIR  DURING  A  SNOWSTORM 


50 

McAdi 


Swann  in  the  same  journal  discusses  new  methods  and  instru- 
ments for  measuring  the  specific  numbers  and  velocities  of  the 
atmospheric  ions,  the  radioactive  content  of  the  atmosphere, 
and  the  potential  gradient. 


202  THE  PRINCIPLES  OF  AEROGRAPHY 

52.  Protection  from  lightning.  Few  questions  have  been 
more  debated  than  the  certainty  of  protection  afforded  by 
lightning  rods  and  protective  devices.  We  may 
sum  UP  the  controversy  in  the  words  of  Kelvin, 
"that  there  is  good  reason  to  feel  that  there  is  a 
very  comfortable  degree  of  security,  if  not  of  absolute  safety, 
given  to  us  by  lightning  conductors  made  according  to  present 
and  orthodox  rules." 

Detailed  instructions  for  proper  installation  of  rods  and 
making  good  grounds  are  given  in  numerous  books,  among 
which  may  be  mentioned  The  Report  of  the  Lightning  Rod 
Conference,  Anderson  on  Lightning  Rods,  Lodge  on  Lightning 
Conductors;  and  McAdie  and  Henry  in  various  Weather 
Bureau  publications;  also  Bulletin  No.  56,  Bureau  of  Standards. 

An  interesting  statistical  study  of  the  ratio  of  buildings 
with  lightning  rods  struck  to  those  not  struck  is  that  made 
by  J.  W.  Smith,  who  ascertained  from  insurance 
companies  that  during  1912  and  1913  of  1,845 
buildings  struck  only  67  were  supplied  with  rods 
or  protective  devices.  Approximately  31  per  cent  of  the 
buildings  insured  were  rodded,  hence  the  number  of  build- 
ings struck  should  have  been  572  instead  of  67,  or  on  these 
figures  the  efficiency  of  the  rods  would  be  about  90  per  cent. 
Five  companies  with  18,000  buildings  insured,  of  which  50 
per  cent  were  rodded,  reported  that  no  building  with  rods 
had  been  burned  or  even  materially  damaged.  Again,  it  is 
noteworthy  that  when  a  rodded  building  is  struck  the  damage 
is  much  less  than  in  the  case  of  an  unrodded  building. 

Briefly,  it  may  be  said  that  all  barns  and  exposed  buildings 
should  have  well-grounded  lightning  rods ;  but  as  for  ordinary 

dwelling  houses  in  city  blocks  there  is  little  like- 
Barns,  spires,     ,.,         .   °.  ,  1     .    .     .          ,        -,     ,  1  11- 

and  exposed      hhood  of  their  being  struck,  hence  rods  may  be  dis- 

houses  should  pensed  with.  The  nature  of  a  locality  determines 
somewhat  the  need;  thus  if  the  building  is  near  a 
water  course,  the  risk  is  greater.  Places  separated  by  a  few 
miles  have  different  frequencies  of  stroke.  Lightning  may 
strike  several  times  in  the  same  place ;  there  is  no  known  rea- 
son why  it  should  not.  It  is  unwise  to  stand  under  or  near 
trees  during  thunderstorms.  Some  trees,  such  as  the  locust 


ATMOSPHERIC  ELECTRICITY 


203 


and  the  pine,  appear  to  be  more  frequently  struck  than  others, 
owing  possibly  to  the  character  of  the  root  system.  How- 
ever, all  trees  may  be  struck. 

PERCENTAGE  OF  DIFFERENT  SPECIES  STRUCK  BY  LIGHTNING  IN 

EUROPE 


Kind  of  tree 

Per  cent 

Kind  of  tree 

Per  cent 

Poplar  
Oak 

30.5 

20  7 

Horse  chestnut  
Plum  

0.3 
.3 

Conifers 

15  5 

Alder          

.2 

Elm 

4  6 

Larch 

2 

Pear 

3.9 

Service  

.2 

Willow 

3  5 

Catalpa        

.1 

Beech 

2  4 

Elder 

.1 

Ash        

1.8 

Lime  

.1 

Linden 

1.8 

Maple  

.1 

Walnut  

1.3 

Vine  

.1 

Cherry  
Apple 

1.2 
1.0 

Others  
Unknown.  ...        

.4 
8.0 

r^Vioc-f  nii-f 

i^nes  inui/  
Birch  
Acacia  

.5 
.4 

100.0 

Plummer  has  discussed  the  liability  of  trees  to  lightning 
stroke1  and  shows  that : 

1.  Trees  are  the  objects  most  often  struck  by  lightning,  because 

(a)  they  are  the  most  numerous  of  all  objects;  (b)  as  a  part 
of  the  earth's  surface  they  extend  upward  and  Wh  treeg 
shorten  the  distance  to  a  cloud ;  (c)  their  spreading  are  struck 
branches  in  the  air  and  spreading  roots  in  the 
ground  present  the  ideal  form  for  conducting  an  electrical  dis- 
charge to  the  earth. 

2.  Any  kind  of  tree  is  likely  to  be  struck  by  lightning. 

3.  The  greatest  number  of  any  species  struck  in  any  locality  is  the 

dominant  species  of  that  locality. 

4.  The  likelihood  of  a  tree  being   struck  by   lightning  is  increased 

(a)  if  it  is  taller  than  surrounding  trees ;  (b)  if  it  is  isolated ; 
(c)  if  it  is  upon  high  ground;  (d)  if  it  is  well  (deeply)  rooted; 
(e)  if  it  is  the  best  conductor  at  the  moment  of  the  flash;  that 
is,  if  temporary  conditions,  such  as  being  wet  by  rain,  transform 
it  for  the  time  from  a  poor  conductor  to  a  good  one. 

5.  Lightning  may  bring  about  a  forest  fire  by  igniting  the  tree  itself, 

or  the  humus  at  its  base.     Most  forest  fires  caused  by  lightning 
probably  start  in  the  humus. 
1  Bulletin  No.  Ill,  Forest  Service. 


204  THE  PRINCIPLES  OF  AEROGRAPHY 

Data  have  been  gathered  both  in  Europe  and  in  the  United 
States  on  the  frequency  of  lightning  stroke  upon  trees  growing 
in  different  soils.  An  average  of  all  results  shows  the  rela- 
tive percentages  to  be: 

Per  cent 

Loam 42 

Sand 22 

Clay 19 

Others,  including  rock,  marl,  and  calcareous  formations.       17 

100 

This  comparison  does  not  take  into  account  the  relative 
areas  of  the  different  soils  covered  in  the  investigations.  A 
fairer  estimate,  made  in  Belgium,  and  reduced  to  actual 
frequency  per  unit  area,  gives  the  following: 

Per  cent 

Loam 23 

Sand 18 

Clay 17 

Others 42 

100 

The  frequency  of  stroke  upon  the  poplar  growing  in  these 
soils  was: 

Per  cent 

On  loamy  soil 28 

On  sandy  soil 24 

On  clay  soil 6 

On  other  soils 42 

100 

The  effects  of  lightning  are  so  remarkable  that  it  is  always 
advisable  to  attempt  to  restore  consciousness  to  a  person  who 
Resuscitation  -^as  been  struck,  even  if  the  case  appears  hope- 
from  lightning  less.  There  are  many  cases  on  record  proving 
the  wisdom  of  such  action.  There  is  good  reason 
for  believing  that  in  the  majority  of  cases  lightning  causes 
suspended  animation  rather  than  somatic  death.  Every 
effort,  therefore,  should  be  made  to  stimulate  respiration  and 
restore  circulation.  Do  not  cease  in  the  effort  for  at  least 
an  hour;  and  be  sure  to  secure  the  services  of  a  physician  as 
promptly  as  possible. 


CHAPTER  XV 

PRECIPITATION 
RAIN 

53.  The  process  that  makes  the  raindrop.  One  gram  of 
pure  water  at  277° A.,  the  temperature  of  maximum  density, 
occupies  one  cubic  centimeter  of  space.  If  we  change  this 
volume  of  water  into  vapor  at  373°  we  find  that  it  expands 
and  now  occupies  a  volume  of  1,698  cubic  centimeters.  When 
expansion  took  place,  work  was  done.  A  reverse  process 
would  be  the  following:  If  1,698  cubic  centimeters  of  vapor 

at  the  given  temperature  were  compressed  into  a      _ 
-  i  .  ,.  .          !  Compression 

drop  of  water  one  cubic  centimeter  in  volume,  or 

about  four  times  the  size  of  a  very  large  raindrop,  a  large  quan- 
tity of  heat  would  be  available  and  there  would  still  remain 
in  the  drop  considerable  internal  energy.  Of  course  rain  is 
seldom  the  result  of  condensation  at  so  high  a  temperature. 

Raindrops    are   not    made   of   pure   water.     Indeed,    it   is 
doubtful  whether,  under  natural  conditions,  we  could  expect 
to  find  a  pure  drop,  and  therefore  some  allowance         Raindrops 
must  be  made  in  applying  the  results  of  labora-         not  pure 
tory  experiments  in  studies  of  condensation  and 
evaporation.     Furthermore,  it  is  seldom  that  we  find  complete 
saturation.     Under  natural  conditions,  even  in  dense  and  wet 
clouds,  the  true  degree  of  saturation  does  not  exceed  95  per 
cent.     And  this  explains  why  it  is  so  hard  to  dry  out  a  mixed 
atmosphere  and  leave  not  a  trace  of  vapor. 

A  molecule  of  water  is  approximately  one  ten-millionth  of  a 
millimeter  in  diameter.     The  size  of  a  fine  drop  of 
rain  is  from  one  one-hundredth  of  a  millimeter  to          raindrops 
half  a  millimeter  in  diameter;  of  a  common  drop, 
from  2  to  4  millimeters;  of  a  large  drop,  from  5  to  7  millimeters. 

The  weight  of  the  largest  raindrop  does  not  exceed  0.2 
gram,  and  its  diameter  is  about  7  millimeters.  Drops  of 
larger  size,  according  to  the  experiments  of  Weisner,  Lenard, 

205 


206  THE  PRINCIPLES  OF  A&ROGRAPHY 

and  others,  when  allowed  to  fall  from  a  height  of  22  centi- 
meters separate  into  small  drops.  Wilson  A.  Bentley,  by 
allowing  raindrops  to  fall  into  a  layer  25  millimeters  deep  of 
fine  uncompacted  flour,  with  a  smooth  surface,  contained  in 
a  shallow  tin  receptacle  about  100  millimeters  in  diameter, 
exposed  to  the  rain  for  about  four  seconds,  obtained  dough 
pellets  which  were  found  by  experiment  to  correspond  very 
closely  in  size  with  the  raindrops  which  made  them.  These 
pellets  were  photographed;  and  the  type  of  storm,  tempera- 
ture, and  approximate  height  of  clouds  noted  and  recorded. 
Three  hundred  and  forty-four  sets  of  raindrop  impressions  were 
secured  in  this  way  from  70  different  storms.  A  remarkable 
discovery  made  in  these  investigations  was  the  astonishing 
differences  in  the  dimensions  of  the  individual  drops  both  in 
the  same  and  in  different  rainfalls.  While  the  larger  ones 
possessed  diameters  of  6  or  8  millimeters,  the  smaller  ones 
were  only  0.8  or  0.6  millimeter,  and  there  were  frequently 
microscopic  drops  too  small  to  make  an  impression.  The 
following  table  shows  the  relative  frequencies.  Of  a  total  of 
867  drops,  there  were : 

149  very  small  drops  (less  than  0.8  mm.),  or  17  per  cent  of  total; 
288  small  drops  from  0.8  to  1.5  mm.,  or  34  per  cent  of  total; 
254  medium  drops  from  1.6  to  3.5  mm.,  or  29  per  cent  of  total; 
141  large  drops  from  3.6  mm.  to  5.1  mm.,  or  16  per  cent  of  total; 
35  very  large  drops  above  5.2  mm.,  or  4  per  cent  of  total. 

Bentley,  in  the  same  manner,  studied  the  distribution  in 
51  storm  areas  and  found  that,  in  general,  the  very  small 
Variation  drops  increase  in  number  from  the  east  edge 
with  storm  toward  the  west,  or  receding  edge,  of  a  storm; 
that  for  other  sizes  there  was  a  progressive 
increase  toward  the  center  of  a  storm,  but  a  decrease  toward 
the  west.  This  law,  however,  does  not  hold  for  thunder- 
storms. It  seems  to  be  a  fact  that,  in  general,  showers  renew 
themselves  or  acquire  new  vigor  within  the  western,  or  reced- 
ing, segment.  Bentley  advances  the  view  that,  ordinarily, 
the  size  of  each  individual  raindrop  depends  largely  upon 
and  increases  with  the  square  of  the  mass  of  upper  and  inter- 
mediate clouds  that  a  drop  passes  through  on  its  journey 
earthward.  Practically  all  of  the  rainfall  from  low  and 


PRECIPITATION 


207 


intermediate  clouds  (low-lying  cumulus,  nimbus,  cirro-stratus, 
and  cirro-cumulus)  consists  of  small  and  medium  drops. 
The  larger  raindrops  are  shed  in  considerable  Heavy  rainfall 
numbers  only  from  thick,  vertically  expanding,  with  electrical 
complex  clouds.  It  was  often  noted  that  when-  1SC  arge 
ever  electrical  discharges  were  unusually  frequent  and  power- 
ful the  rainfall  was  unusually  thick  and  heavy  and  consisted 
of  raindrops  of  all  sizes,  the  larger  sizes  predominating.  The 
following  table  compares  the  sizes  of  drops  that  fall  during 
lightning  and  when  there  is  no  lightning : 


Near  lightning     |  Distant  lightning 

No  lightning 

Very  small  drops  .  . 
Small  drops  
Medium  drops  .... 
Large  drops  
Very  large  drops  .  . 

15 

61 
64 
58 
23 

31 
67 
63 
23 
3 

38 
70 
57 
22 
1 

Bentley  thinks  that  the  major  portion  of  the  rainfall  of 
thunder  showers  is  of  snow  origin;  also  that  a  very  large 
portion  of  the  rainfall  in  all  climates  is  due  to  the  melting 
of  snowflakes  or  granular  snow. 

54.  The  rain  gauge.  The  invention  of  the  rain  gauge  is 
attributed  to  an  Italian,  Benedetto  Castelli,  who  in  June, 
1639,  informed  Galileo  that  he  had  measured  the  rainfall 
with  a  vase  one  spanne  in  depth  and  half  a  spanne  in  diameter. 
But  Dr.  Y.  Wada,  director  of  the  Korean  Meteor- 
ological Observatory,  has  shown  that  rain 
gauges  were  in  use  in  Korea1  as  early  as  1442. 2 
The  dimensions  of  the  ancient  gauge  were:  depth,  30  centi- 
meters; diameter,  14  centimeters.  A  gauge  of  the  period  of 
1770  was  found  by  Wada  on  an  inspection  trip,  actually  in  use 
at  the  observatory  of  Chemulpo.  On  the  granite  pillar  of 
the  gauge  represented  in  the  illustration  (Fig.  80)  found  at 
Taiku  are  engraved  three  large  Chinese  characters  meaning 
"Instrument  to  measure  rain,"  and  seven  smaller  characters 
meaning  "Constructed  in  the  fifth  month  of  the  cycle  of  the 
year,"  a  date  in  the  Chinese  calendar  corresponding  to  1770. 

1The  modern  name  for  Korea,  now  a  department  of  Japan,  is  Chosen. 
2  Jour,  of  the  Met.  Soc.  of  Japan,  Mar.,  1910;  also  Quart.  Jour,  of  the  Royal 
Met.  Soc.,  Jan.,  1911. 


From  Quart.  Jour   of  the  Royal  Met. 

FIG.  80.     THE  OLDEST  RAIN  GAUGE 
This  rain  gauge  was  erected  at  Taiku  in  1770. 


..  1911 


208 


PRECIPITATION  209 

This  is  evidence  that  rain  observations  began  in  Korea  two 
centuries  before  they  were  made  in  Europe,  and,  what  is 
more  remarkable,  there  appears  to  have  been  an  udometric 
survey,  or  network  of  rain-reporting  stations.  The  reports 
were  probably  used  in  connection  with  the  rice  crop.  A  large 
amount  of  water  is  required  in  the  cultivation  of  this  crop, 
especially  in  the  first  period  of  its  transplanting;  and  should 
the  rainy  season  not  commence  until  the  middle  of  July,  and 
the  rainfall  be  deficient,  the  crop  may  be  injured.  It  is  said 
that  during  droughts  from  the  earliest  times  the  rulers  of 
Korea  caused  prayers  for  rain  to  be  offered  to  the  saints  of 
the  mountains  and  rivers.  This  is  a  striking  illustration  of  the 
dependence  of  a  community's  welfare  upon  rainfall.  Dr.  Wada 
states  that  in  his  judgment  it  was  thus  through  necessity  that 
the  rain  gauge  was  invented. 

Various  types  of  gauges  are  used  in  different  parts  of  the 
world.     In  Europe  and  Asia  gauges  are  of  glass  or  porcelain, 
with  metal  guards.     The  rain  collected  is  poured 
out    and   measured   in    a    small    glass    graduate.       gauge™ 
The  ratio  of  the  area  of  the  receiving  surface  to 
the  area  of  the  measuring  tube  must  be  accurately  determined. 
Perhaps   the   most   satisfactory   rain   gauge   is   the   so-called 
British  gauge,  which  has  a  galvanized  iron  funnel  about  127 
millimeters  in  diameter. 

It  is  of  a  pattern  similar  to  the  Snowdon  gauge.  The 
Snowdon  gauge  probably  originated  with  Dalton.  It  is 
constructed  almost  exclusively  of  copper  or  galvanized 
iron;  the  funnel  is  fitted  with  a  ring  of  turned 
brass.  The  British  rain  gauge,  with  respect  to  don  gauge" 
funnel,  brass  ring,  inner  can,  and  glass  bottle,  is 
identical  with  the  Snowdon  gauge;  the  outer  can  of  the 
former,  however,  is  made  of  a  cheap  combination  of  lead 
and  iron.  Should  the  outer  can  become  damaged  or  leaky, 
the  efficiency  of  the  gauge  is  not  thereby  impaired.  This 
gauge  has  a  glass  bottle,  a  graduate  glass  measure,  and  is 
certified  by  the  British  Rainfall  Organization,  an  organiza- 
tion founded  by  the  late  G.  J.  Symons.  The  society  collects 
records  of  rainfall  from  nearly  5,000  voluntary  observers. 
In  the  United  States  many  different  gauges  have  been  used. 

15 


210  THE  PRINCIPLES  OF  A&ROGRAPHY 

The  Weather  Bureau  uses  a  three-part  metal  gauge  some- 
what larger  than  other  gauges.  There  is  a  galvanized  iron 
top-piece  with  brass-turned  edge,  and  funnel 
Bur^atf gauge^  snaPecL  This  fits  as  a  sleeve  on  a  cylinder 
of  galvanized  iron.  The  diameter  of  the  col- 
lecting surface  is  about  203  millimeters  and  the  length  of 
the  cylinder  about  510  millimeters.  This  cylinder  serves 
in  case  of  overflow  to  retain  excess  rain;  and  is  also 
available  for  snow  measurements.  The  third  piece  of  the 
gauge  is  a  copper  or  brass  cylinder  with  a  diameter  of  about 
64  millimeters.  The  rainfall  is  collected  and  measured  in 
this  inner  tube.  The  area  of  the  collecting  surface  is  about 
a  thousand  square  millimeters,  or  ten  times  the  area  of  the 
inner  measuring  surface.  The  catch  of  rain  is,  therefore, 
magnified  ten  times.  A  wooden  stick,  generally  cedar,  is 
graduated  in  millimeters  or  inches,  and,  proper  allowance 
being  made  for  wetting  and  displacement,  the  depth  of  rain 
is  read  on  the  wet  stick.  One  millimeter  will  represent  one 
tenth  of  a  millimeter  of  rainfall.  There  are  many  forms  of 
automatic  and  self -registering  gauges,  some  in  which  the 
weight  of  the  water  is  made  to  record  on  a  moving  drum,  as 
in  the  well-known  Fergusson  gauge  and  its 
various  modifications.  In  other  types,  such  as 
the  tipping-bucket,  a  definite  quantity  of  rain 
tips  a  small  bucket,  which  in  tipping  makes  electric  contact 
and  thus  records  at  a  distance.  All  such  gauges  need 
frequent  inspection  and  cleaning. 

To  insure  accuracy  of  the  catch,  a  gauge  should  be  exposed 
with  the  collecting  area,  if  possible,  level  with  the  ground  or 
a  foot  above.  There  should  be  no  near  obstructions  and  no 
trees  or  shrubbery  close  by  from  which  water  could  drip  or 
be  blown  into  the  gauge.  Many  of  the  Weather 
Bureau  gauges  are  exposed  on  the  roofs  of  tall 
buildings  in  cities  and  are  subject  to  wind  eddies. 
The  catch  at  nearly  all  the  principal  cities  is  thus  too  low, 
differing  by  30  to  40  per  cent  from  the  amount  which  would 
be  collected  by  a  gauge  on  the  ground.  Other  inaccuracies 
are  met  unless  the  gauge  is  provided  with  some  form  of  wind 
screen,  such  as  that  devised  by  Professor  Nipher.  There  is 


PRECIPITATION  211 

still  another  cause  of  inaccuracy.  In  many  cities  the  place 
of  exposure  has  been  changed  so  often  by  removals  that 
the  data  are  not  comparable.  As  an  offset, 


certain  nonofficial  records  have  been  maintained  exposure 

with   great   fidelity,    over   long   periods,   without  should  not 

1  £  ^     j      r  be  changed 

change  of  exposure  or  method  of  measurement. 

Records  have  been  maintained  at  New  Haven  for  a  period 
of  128  years;  at  New  Bedford,  103;  at  Albany,  94;  at  New 
York,  90;  at  San  Francisco,  64;  and  at  Blue  Hill,  32. 

Rainfall  should  be  measured  as  soon  as  possible  after  the 
rain  has  ended.  The  best  results  are  not  obtained  by 
measuring  once  in  twenty-four  hours.  Rain  is  lost  through 
evaporation;  and  light  showers  are  not  recorded  on  many 
self-registering  gauges,  the  mechanism  not  being  sufficiently 
sensitive.  Thus  the  time  precipitation  begins  and  the  time 
it  ends  is  not,  with  certainty,  known  if  dependence  is  placed 
upon  records  made  by  such  instruments.  At 
Blue  Hill  Observatory  an  instrument  known  as  Jmbroscope 
an  ombroscope,  devised  by  Fassig  and  modified 
by  Fergusson,  shows  by  the  discoloration  of  prepared  paper 
moved  by  clockwork  the  time  when  the  first  drop  falls.  The 
wind  carries  the  smaller  raindrops  past  the  opening,  and  it 
may  be  that  only  the  time  of  falling  of  the  heavier  drops  is 
thus  recorded.  The  instrument  does  not  record  the  begin- 
ning or  ending  of  fog.  Nor  does  any  known  type  of  rain 
gauge  give  a  true  record  if  the  raindrops  are  small  and  the 
wind  high;  for  these  fine  drops  are  easily  carried  past  and 
even  whirled  out  of  the  mouth  of  the  gauge.  There  is  room 
for  improvement  in  this  direction. 

55.  Variation  of  rainfall  with  altitude  in  mountainous 
countries.  It  is  evident  that  mountains  materially  influence 
the  rainfall.  Various  writers  have  shown  the  effect  of 
mountain  ranges  in  causing  an  uplift  of  the  air  current  and 
subsequent  condensation  and  precipitation.  Cloud-capped 
summits  are  familiar  throughout  the  world,  and  even  moder- 
ate elevations  are  sometimes  covered  with  clouds  when  the 
lowlands  have  sunshine.  Particularly  noticeable  are  the 
cloud  caps  of  high  mountains  or  snow  peaks.  In  these  alti- 
tudes the  air  is  cooled  below  the  temperature  of  saturation, 


212  THE  PRINCIPLES  OF  AEROGRAPH? 

and  cloud  or  snow  banners  form,  streaming  out  from  the 
peak  in  the  direction  from  which  the  wind  blows, —  "in 
Cloud  the  teeth  of  the  wind,"  as  it  were.  Snow  banners 

banners  and  in  the  Sierra  are  eloquently  described  by  Muir. 
cloud  caps  In  addition  to  these  forms  there  are  the  well- 
known  "Tablecloth"  at  Cape  Town,  and  the  "Helm  and 
Bar"  at  Cross  Fell.  Perhaps  the  most  elaborate  attempt  to 
discuss  mathematically  the  condensation  of  vapor  on  moun- 
tain slopes  is  that  of  Pockels,  who  deduces  the  minimum 
elevation  at  which  condensation  may  begin.  Assuming  the 
average  vertical  distribution  of  temperature  and  moisture  for 
each  of  the  four  seasons,  he  finds  that,  starting  from  an  eleva- 
tion of  0  meters,  the  following  are  the  approximate  heights: 

MINIMUM  ELEVATIONS  FOR  CONDENSATION 


Elevation  in 
meters 

Spring 

Summer 

Autumn 

Winter 

0 
500 
1000 
2000 
3000 
4000 

725 

485 
855 
920 
830 
700 

850 
710 
570 
730 
1060 
1125 

405 
615 
600 
1180 
1208 
1240 

400 
760 
1070 
1100 
1130 
1100 

The  intensity  of  condensation,  and  presumably  precipi- 
tation, is  greatest,  Pockels  thinks,  where  the  slope  is  steepest. 
This  conclusion  is  not,  however,  in  accord  with  facts,  and 
Factors  there  are  factors  other  than  merely  those  of  eleva- 

affecting  ^         tion  and  the  ascent  of  the  air  affecting  the  inten- 
precipitation      sity  of  precipitation-     in   California  the  author 

has  found  a  definite  increase  in  precipitation  with  elevation 
in  the  Sierra  Nevada  in  going  east  from  the  Sacramento 
Valley  and  the  San  Joaquin  Valley.  In  the  lowlands  the 
rainfall  is  rather  evenly  distributed,  and  on  the  same  level 
the  distribution,  as  to  both  intensity  and  frequency,  is  com- 
paratively uniform.  There  is,  however,  a  marked  difference 
Elevation  of  'm  ^ne  amount  of  rainfall  at  stations  close  together 
maximum  but  differing  in  elevation.  The  amount  of  rain 
increases  as  one  goes  from  the  floor  of  the  valley 
through  the  foothill  section  and  up  the  mountain  side, 
reaching  a  maximum  at  a  height  of  about  1,500  meters. 


PRECIPITATION  213 

The  records  of  the  stations  along  the  line  of  the  railroad  from 
Sacramento  to  Summit,  covering  a  period  of  about  forty 
years,  show  a  steady  increase  in  the  quantity  of  rain  caught 
by  the  gauges  of  about  75  millimeters  for  every  100  meters 
rise  in  elevation.  The  rate  of  increase  is  greatest  about  the 
1,000-meter  level,  and  becomes  negative  above  the  1,500- 
meter  level.  At  these  high  levels,  however,  much  of  the 
precipitation  falls  in  the  form  of  snow,  and  it  is  possible  that 
with  our  present  methods  of  reduction  true  values  have  not 
been  obtained. 

East  of  the  Sierra  crest  precipitation  decreases  rapidly 
with  decrease  in  altitude,  maintaining  a  constant  rate  to  the 
1,500-meter  level  and  a  diminishing  rate  below  Rainfan  east 
this  elevation.  The  distance  and  precipitation  of  the  Sierra 
curves  conform  to  the  profile  in  general  shape,  crest 
except  that  their  maxima  are  west  of  the  topographic  crest, 
occupying  the  same  relative  position  with  respect  to  the  Great 
Valley  as  the  1,500-meter  level.  They  have  a  tendency  to  be- 
come horizontal  over  the  level  portion  of  the  profile,  to  rise  over 
western  slopes  below  the  1,500-meter  contour,  to  fall  over  west- 
ern slopes  above  this,  and  to  fall  over  eastern  slopes.  In  other 
words,  the  general  slope  of  the  country  seems  to  have  more 
to  do  with  the  amount  of  precipitation  than  does  altitude. 

Precipitation  upon  the  plains  of  Northern  India  and  the 
southern  slope  of  the  Himalayas  exhibits  a  similar  variation. 
An    empirical    equation    giving    the    relation    of 
precipitation  and  elevation  has  been  developed          upontiie 

from    observations    in    that    region,    as    follows  :          plains  of 

Northern 

R=  1  +  1.92/&-0.40  fc2  +  0.02fc8,  India 


in  which  R  represents  the  amount  of  rain  and  h  the  relative 
height  in  units  of  a  thousand  feet  above  an  assumed  plane, 
which  was  itself  300  meters  above  sea  level.  The  critical 
elevation  was  1,270  meters  above  sea  level,  and  observations 
were  sufficient  to  determine  that  the  form  of  the  curve  above 
this  elevation  was  similar  to  that  below,  the  complete  curve 
approximating  a  cubic  parabola  whose  axis  is  the  line  repre- 
sented by  the  critical  elevation. 

Wilhelm  Krebs  called  the  author's  attention  to  the  results 


214  THE  PRINCIPLES  OF  AEROGRAPH  Y 

of  measurements  made  by  him  on  the  Storm  and  Draken 
mountains  in  South  Africa,  published  in  the  Deutsche  Rund- 
In  Africa  schau  fur  Geographic  und  Statistik,  August,  1890, 
and  in  and  September,  1908.  Krebs  applies  the  same 

process  to  the  Sacramento  and  San  Joaquin  rec- 
ords, using  as  the  base  the  valley  floor  from  Sacramento 
to  Stockton.  He  finds  that  the  ratio  of  Auburn  elevation  to 
Gold  Run  elevation  is  as  1  to  2.4;  the  precipitations  as  1 
to  1.6;  and  the  gradients  of  the  slope  as  1  to  1.5.  The  ratio 
of  Auburn  and  Blue  Canon  is  as  1  to  3.6;  the  precipitations 
as  1  to  2.1;  and  the  gradients  of  the  slope  as  1  to  1.9.  He 
also  compares  Mokelumne  Hill,  West  Point,  and  Bear  Valley. 
The  ratio  of  precipitation  agrees  with  the  gradients  of  slope 
and  differs  from  the  ratio  of  elevations. 

Probably  the  best  rainfall  record  of  exceptionally  heavy 
precipitation  is  that  made  at  Baguio,  Philippine  Islands,  July 
14-15,    1911.     The  chart   (Fig.   81)   taken  from 
^e  Manila  Weather  Bulletin  shows  that  the  total 


heavy  ranfall 

rainfall  from  noon,  July  14,  to  noon  of  the  next 

day  was  1,168  millimeters  (45.99  inches).  The  greatest  hourly 
amounts  were  91  and  90  millimeters;  the  greatest  ten-minute 
rainfall  was  18  millimeters;  and  the  greatest  five-minute  fall 
was  10  millimeters.  The  total  precipitation  at  Baguio  for  the 
four  days,  July  14-17,  was  2,239  millimeters  (88.15  inches). 

The  heaviest  monthly  rainfall  in  the  United  States  occurred 
at  Helen  Mine,  Cal.,  in  January,  1909,  when  1,817  milli- 
H  .  meters  (71.54  inches)  fell.  The  heaviest  twenty- 

rainfalls  four-hour  rainfall  in  the  United  States  occurred 

n  *?erf  «?t  at  AltaPass>  Mitchell  Co.,  N.  C.,  when  564.4  mm. 
fell  from  2  P.M.  July  15  to  2  P.M.  July  16,  1916. 
The  heaviest  short-period  rainfall  occurred  during  a  cloud- 
burst at  Campo,  Cal.,  August  12,  1891,  when  292  millimeters 
fell,  practically  in  eighty  minutes. 

The  table  on  p.  216  gives  the  rate  and  duration  of  the 
heaviest  known  rainfalls. 

56.  Measuring  rainfall  by  rings  of  annual  growth.  Many 
attempts  have  been  made  to  determine  secular  variation  in 
rainfall  by  studying  the  rings  of  annual  growth  on  trees. 
Manson  and  others  have  made  cross-sections  of  the  Sequoia 


PRECIPITATION 


215 


216 


THE  PRINCIPLES  OF  AEROGRAPHY 


gigantea,  also  of  the  redwoods  of  California,  and  have  examined 
the  consecutive  character  of  the  rings  with  reference  to  the 
known  rainfalls  and  thus  tried  to  discover  if  there  were  any 
periodicity  in  rainfall.  Huntington  has  also  made  extensive 


Place 

Date 

Rate 
per 
hour 
in  mm. 

Actual 
duration 
of  rate 
in  hours 

Baguio,  P.I  

14,    VII,  1911 

49 

24.0 

Cherrapunji,  Khasi  Hills,  India*  . 
Montell,  Tex  

14,      VI,  1876 
28,      VI,  1913 

43 

28 

24.0 
18.5 

Concord,  Pa  
Campo,  Cal  

5,  VIII,  1843 
12,  VIII,  1891 

135 

219 

3.0 
1.3 

Guinea,  Va  
Curtea  de  Arges,  Rumania  
Graz,  Austria  
Galveston,  Tex  

24,  VIII,  1906 
7,    VII,  1889 
3,    VII,  1914 
4,      VI,  1871 

470 
615 
110 
429 

0.5 
0.3 
0.5 
0.23 

Fort  McPherson,  Neb  
Kansas  City  Mo 

27,       V,  1868 
6      IX   1914 

428 
195 

0.083 
0  083 

Valdivia,  Chile  
Malta 

11,      VI,  1912 
16        X   1913 

480 
51 

0.0083 
3  0 

Amboina  (Dutch  Indies)  
Rogodjampi  (Dutch  Indies)  .... 
Patjet  (Dutch  Indies)  

6,      VI,  1912 
10,       II,  1912 
14,   XII,  1912 

120 
192 
204 

0.083 
0.083 
0.083 

*Blanford,  in  his  Climates  and  Weather  of  India,  1889,  pp.  77  and  265,  gives 
1,036  mm.  at  Cherrapunji  as  the  heaviest  rainfall  recorded  for  one  day  in  India. 

studies  along  more  general  lines.  A.  E.  Douglas,  of  the 
University  of  Arizona,1  has  identified  a  long  period  of  tree 
growth  (Arizona  pines)  with  certain  meteor- 
°l°gical  cycles.  There  are  four  marked  maxima 
about  the  years  1400,  1560,  1710,  and  1865. 
A  period  of  33.8  years  fits  very  well  after  1730.  The  last 
crest  came  in  1900,  and  this  can  be  identified  with  the  well- 
known  Bruckner  period.  There  is  a  rather  persistent  period 
of  about  21  years,  its  last  crest  occurring  in  1892.  This  pul- 
sation is  well  marked  from  1400  to  1520;  then  for  a  hundred 
years  fails  or  shows  discrepancies,  and  after  that,  from  1610 
to  the  present  time,  is  marked  and  regular.  There  is  also  a 
period  of  11.4  years  which  is  practically  the  sunspot  cycle. 

1  "Method  of  Estimating  Rainfall  by  the  Growth  of  Trees,"  Am.  Geog.  Soc. 
Bull,  May,  1914. 


PRECIPITATION  217 

Correlation  between  tree  growth  and  sunspot  variation  is 
not  confined  to  the  trees  of  Arizona  and  California,  for  meas- 
urements made  on  thirteen  tree  sections  from  the  Tree  growtn 
forest  of  Eberswalde  indicate  a  relation  between  and  sunspot 
tree  growth  and  rainfall,  temperature,  and  solar  variatlon 
conditions.  Apparent  climatic  cycles  have  been  investigated, 
and,  what  is  of  real  importance,  a  method  of  estimating  rain- 
fall has  been  found  which  may  be  susceptible  of  extension 
and  adaptation  to  other  fields  of  science. 

57.  Rainfall  distribution.  Isohyets  are  lines  denoting  equal 
amounts  of  rainfall,  and  until  within  a  year  or  two  they 
have  been  the  only  lines  used  in  charting  rainfall.  Igoh  etg 

Isomers  are  lines  of  equal  proportion  of  rainfall; 
that  is,  lines  which  show  the  rainfall  of  a  given  period  in 
percentages  of  the  total  rainfall.     But  in  all  rainfall  maps 
thus  far  made,   no  correction  has  been  applied  isomers 

for  either  elevation  or  temperature.  The  first 
rainfall  map  was  published  in  the  Physical  Atlas  of  Berghaus 
in  1845,  and  showed  isohyets  for  Europe.  Loomis,  in  1882, 
drew  the  first  chart  of  world  isohyets,  and  these  were  sub- 
sequently redrawn  by  Buchan.  About  1898  Supan  issued 
his  charts.  Herbert  son  soon  followed,  the  latter  being  the 
first  to  give  monthly  values.  In  Bartholomew's  Atlas  may 
be  found  the  most  comprehensive  series  of  rain  maps.  From 
such  charts  it  may  be  seen  that  the  equatorial  regions  in 
general  have  a  precipitation  amounting  to  about  Rainfall  in 
1,000  millimeters.  Eastern  coasts  receive  a  equatorial 
relatively  heavy  rainfall,  especially  if  moun-  reg101 
tainous.  The  rain  is  heaviest  where  the  trade  or  monsoon 
winds  blow  directly  toward  the  land.  At  points  from  20  to 
35  degrees  either  side  of  the  equator  there  is  a  marked  decrease 
in  rainfall,  and  dry  zones  and  deserts  appear  west  of  the 
regions  influenced  by  the  trades  and  monsoons. 

There  is,  on  the  whole,  a  steady  diminution  of  rainfall  from 
equator  to  pole  corresponding  with  the  diminution  of  temper- 
ature and  of  vapor-carrying  capacity  of  the  air.  Decrease 
Three  exceptions  should  be  noted:  (1)  The  poleward 
coastal  lands,  where  sudden  changes  of  temperature  are  fre- 
quent and  the  air  is  nearly  saturated  with  moisture,  are  rainy, 


218  THE  PRINCIPLES  OF  AEROGRAPHY 

except  where  cold  currents  well  up  and  make  a  cool  area  near 
the  coast,  as  happens  near  the  tropics  on  the  west  coasts. 
(2)  Great  temperature  changes  also  occur  in  mountain  lands, 
and  where  the  air  is  sufficiently  damp,  rain  is  common.  (3)  The 
hearts  of  the  continents,  far  from  the  source  of  water  vapor 
in  the  oceans,  and  the  regions  reached  by  winds  blowing  out 
from  them  over  dry  land,  are  very  dry.  There  is  a  great 
area  of  excessive  rainfall  (over  2,000  millimeters)  over  the 
Atlantic  between  Newfoundland  and  Ireland;  and  a  more 
restricted  area  on  the  northwest  coast  of  the  United  States, 
Areas  of  including  Alaska.  From  Cape  Flattery  to  Cape 

excessive  Mendocino  the  annual  precipitation  ranges  from 
2,500  to  3,000  millimeters.  Southward  from  Cape 
Mendocino  the  rainfall  decreases;  near  San  Francisco  it  is 
about  600  millimeters;  and  at  San  Diego  about  250  milli- 
meters. Types  and  percentages  of  rainfall  in  the  United 
States  are  shown  in  Fig.  82. 

The  term  ''norm"  is  frequently  used  in  connection  with 
rain  to  represent  the  amount  that  would  be  precipitated 
The  "norm M  provided  the  rain  were  uniformly  distributed 
or  normal  throughout  the  period  under  consideration. 
Thus,  if  the  total  annual  fall  is  divided  by  365 
and  the  quotient  multiplied  by  the  number  of  days  in  each 
month,  we  get  the  norm  for  the  month.  If,  as  is  the  practice 
with  some  writers,  a  value  of  100  is  given  to  the  norm,  then 
the  amounts  can  be  expressed  in  percentages.  Thus,  if  at  a 
given  place  the  norm,  or  normal  rainfall,  for  January  is  2.5 
millimeters,  then  for  a  month  with  3  millimeters  the  cor- 
responding norm  would  be  120. 

The  term  ' ' pluviometric  coefficient"  was  introduced  by 
Angot  to  indicate  the  ratio  of  the  mean  daily  rainfall  of  a 
The  particular  month  to  the  mean  daily  rainfall  of 

"pluviometric  the  whole  year.  Wallis1  uses  the  term  "equi- 
pluves"  for  the  lines  of  equal  departure  from 
the  rainfall  norm.  Mill  uses  the  term  "splash"  to  represent 
"Splash,"  the  distribution  of  rain  in  a  shower,  while  the 
"smear"  term  "smear"  represents  the  generalization  of  a 

succession  of  splashes.  More  appropriate  terms  are  needed. 

1  Monthly  Weather  Review,  Jan.,  1915. 


PRECIPITATION 


ininm    irwrPHi ini  i innninni  11  \m \\  irwni inpinnHnpwnni 


MINIMI 


*r*  Orleans  Z/ct. 


iiini     iripwiitinnnnni  11 11  innnni^  innni     innnnBpnnm  11  ni  innni 


From  Bulletin  Q,  U.  S.  Weather  Bureau 

FIG.  82.     TYPES  OF  MONTHLY  DISTRIBUTION  OF  PRECIPITATION 
IN  THE  UNITED  STATES 


220  THE  PRINCIPLES  OF  A&ROGRAPHY 

In  addition  to  charts  of  rainfall  for  seasons  and  months, 
Mill,  Reed,  and  others  have  attempted  to  chart  the  distri- 
Cyclonic  unit  bution  °f  rainfall,  using  the  cyclone  or  barometric 
for  rainfall  depression  as  a  unit.  The  need  of  some  unit 
distribution  otner  than  a  purely  arbitrary  time  division  has 
been  advocated  by  Ward,  and  the  cyclonic  unit  seems  suitable 
for  many  purposes  in  climatological  work.  The  term  "smear," 
which  Mill  introduced  in  his  study  of  rainfall  distribution 
accompanying  the  passage  of  cyclones  across  the  British 
Isles,  has  been  used  by  Reed  in  discussing  cyclonic  rain  for 
certain  storms  crossing  the  United  States.  While  the  smears 
for  British  cyclones  show  large  continuous  areas  for  rainfall 
of  25-millimeters  depth,  such  continuity  is  not  so 
rainfall  we^  marked  in  the  smears  for  the  United  States. 

There  does,  however,  seem  to  be  a  relation  between 
areas  of  heavy  precipitation  and  extensive  water  areas,  such 
as  the  Great  Lakes,  the  Gulf  of  Mexico,  the  Mississippi 
watershed,  and  the  Atlantic.  But  there  are  many  instances 
of  areas  of  heavy  precipitation  being  at  some  distance  from 
the  track  of  the  cyclone. 

It  should  be  pointed  out,  however,  that* in  such  work  as 
the  foregoing  the  storm  tracks,  as  charted,  are  not  necessarily 
the  true  paths  of  disturbances,  but  only  approximations. 
Locating  the  area  of  lowest  pressure  is  not  the  best  way  to 
find  a  storm  center,  if  by  the  latter  term  we  mean  the  center 
of  air  motion.  In  fact,  the  methods  used  for  charting  storms 
are  practically  those  of  thirty  years  ago.  If  we  determine 
and  represent  the  progressive  movement  of  the  storm  by 
charting, the  air  motion,  laying  emphasis  on  wind  rather  than 
pressure,  we  may  get  more  accurate  storm  tracks.  The 
following  is  a  suggestion  given  by  Bjerknes,  in  a  lecture 
before  the  University  of  London:  Chart  the  winds  so  as  to 
show  the  horizontal  direction  of  flow;  then  represent  velocity 
by  separate  lines  crossing  the  lines  of  flow  at  right  angles, 
indicating  the  speed  by  comparative  proximity.  Such  a 
chart  will  show  lines  of  convergence  and  divergence,  useful 
in  studying  cyclonic  or  anticyclonic  motion.  If  topography 
is  also  shown,  there  will  obviously  be  ascending  currents 
where  the  winds  blow  toward  high  levels,  and  descending 


PRECIPITATION  221 

currents  on  the  leeward  slopes.  Comparing  charts  thus 
obtained  with  actual  precipitation,  it  appears  that  the  heaviest 
rainfall  is  where  there  are  uprising  currents,  and  the  lightest 
where  there  are  descending  currents. 

SNOW 

58.  Snow  crystals.  Perhaps  the  most  comprehensive  and 
at  the  same  time  most  detailed  study  of  snow  crystals  is  that 
made  by  W.  A.  Bentley  of  Jericho,  Vermont.  His  collection 
includes  more  than  1,000  photomicrographs  of  snow  crystals 
obtained  during  a  period  of  seventeen  years  of  observation. 
He  has  classified  snow  crystals  according  to  form  and  accord- 
ing to  position  relative  to  the  storm  center.  He  adopts 
Hellmann's  general  classification,  dividing  them  into  colum- 
nar and  tabular;  and  for  convenience  divides  these  into 
subvarieties,  using  names  suggested  by  Scoresby.  Solid 
tabular  forms  are  called  lamellar,  while  crystals  Types  of 

of  more  or  less  open  structure  possessing  solid  snow 

tabular  nuclei  resembling  ferns,   are  called  fern  crystals 

stellar.  Columnar  forms  connecting  one  or  more  tabular 
crystals  are  called  doublets;  and  extremely  long,  needle-shaped 
types,  needilar  or  spicular. 

During  cold  snowfalls  the  solid  columnar  and  tabular 
forms  appear  to  be  of  nearly  equal  number  with  the  more 
open  stellar  and  fernlike  varieties,  and  they  considerably 
outnumber  the  granular  forms.  Doubtless  the 

1  4.-  -u  r  j        •  r        Slze  of  snow 

actual  connection  between  forms  and  sizes  of  crystals  and 
snow  crystals  and  the  temperature  of  the  air  is  temperature 
more  intimate  than  our  present  knowledge  would 
indicate,  for  our  studies  are  based  on  air  temperatures  at  the 
earth's  surface  instead  of  at  the  cloud  levels  where  the  crystals 
form.  By  close  study  of  the  microphotographs,  Bentley 
finds  that  the  most  common  forms  outlined  within  the  nuclear 
parts  of  the  crystals  are  a  simple  star  of  six  projecting  angles, 
a  solid  hexagon,  and  a  circle.  The  subsequent  additions 
assume  a  bewildering  variety  of  shapes,  each  of  which  usually 
differs  widely  from  the  one  that  preceded  it  and  from  the 
primitive  nuclear  form  at  its  center.  Examining  the  photo- 
graph (Fig.  83a),  we  see  a  crystal  star  as  it  was  probably 
shaped  at  birth.  In  this  form  it  was  probably  carried  upward 


222 


THE  PRINCIPLES  OF  AEROGRAPHY 


FIG.  83.  h  SNOW  CRYSTALS 


I      After  Bentley 


by  ascending   currents,    assuming   at   some  upper  level   the 
solid  hexagonal  form  around  the  star-shaped  nucleus.     Becom- 
ing heavier,    it   probably   fell,   acquiring  further 
cr°yS3s0n  °f     growth  in  lower  levels.     The  crystal  in  Fig.  836 
probably  originated  at   a  high   level   and   com- 
pleted its  growth  at  low  levels. 

The  magnification  in  Bentley 's  photographs  varies  from 
30  to  50  diameters.  The  author  in  studying  the  structure  of 
snowflakes  found  that  the  most  frequent  form  was  a  needle 
or  spicula.  In  general  the  angles  formed  by  the  crossing  of 
these  needles  were  60°  or  120°.  (Figs.  84  and  85.)  The  ratio 
of  the  depth  of  snow  to  water  at  273°  A.  was  14  to  1  of  the 
flakes  in  Fig.  84. 


PRECIPITATION  223 

59.  Measurements  of  snowfall.  It  may  be  frankly  con- 
ceded that  many  of  the  measurements  of  snow  are  of  doubtful 
accuracy.  It  is  problematical  whether  the  snow  gauge  of 
ordinary  design,  owing  to  clogging  and  wind  action,  catches 
the  proper  proportion  of  the  fall.  Where  possible,  snow 
should  be  melted  and  converted  into  water  immediately  upon 
reaching  the  collecting  surface;  but  this  is  impracticable  at 
most  points.  Snow  collected  in  a  gauge  is  usually  brought 
into  a  warm  place,  melted,  and  then  measured.  Warm 
water  may  be  used,  if  an  accurate  estimate  of  Water 
the  snow  equivalent  is  to  be  ascertained.  In  equivalent 
general,  one  inch  of  snow  is  considered  the  of  snow 
equivalent  of  one  tenth  of  an  inch  of  water,  but  this  relation 
is  not  fixed,  and  the  writer  has  found  that  in  the  same  snow 
bank  the  density  of  the  snow  varies  markedly.  The  snow 
sampler  devised  by  Church,  referred  to  below,  and  a  form  of 


% 


FIG.  84.     SMALL  SNOWFLAKES 
Magnified  20  diameters. 


224  THE  PRINCIPLES  OF  ARROGRAPHY 


t     1 


FIG.  85.     STRUCTURE  OF  A  SNOWFLAKE  McAdie 

Magnified  200  diameters. 

density  gauge,  in  which  the  given  volume  of  snow  is  weighed 
on  a  spring  balance,  enable  more  accurate  determination  of 
the  water  content  of  snow  than  other  gauges.  An  attempt 
made  several  years  ago  by  the  United  States  Weather 
Bureau  to  use  snow  bins  was  not  successful. 

Brooks  has  studied  the  distribution  of  snow  in  two  great 
snowstorms  and  finds  that,  in  general,  the  snowfall  is  spread 
over  a  wide  area  on  each  side  of  the  storm  track;  the  heaviest 
Distribution  snow  comes  with  northeast  winds  and  occurs  in  a 
of  snow  belt  about  100  to  200  miles  north  of  the  track;  the 

northwest  winds  in  the  southwest  quadrant  sprinkle 
light  snowfall  over  the  country  to  a  distance  of  about  300 
miles  south  of  the  track  of  the  cyclone  center.  The  effects  of 
local  topography  and  geography  make  the  distribution  patchy. 

In  some  of  the  western  states,  notably  California,  Nevada, 
Utah,  Idaho,  and  Arizona,  it  is  important  to  determine  the 
probable  water  supply  from  the  snow  cover.  For  this  a  snow 


PRECIPITATION  225 

scale,  or  stake,  supplemented  by  a  few  density  measurements, 
is  used.  Unless,  however,  something  is  known  of  the  rate  of 
melting  and  the  losses  through  evaporation,  esti- 
mates based  on  such  fragmentary  data  must  be 
taken  with  due  allowance  for  error.  Perhaps  the 
most  reliable  data  on  snow  measure  in  the  United  States  are 
those  made  in  California,  Nevada,  Utah,  and  Colorado. 

For  a  distance  of  nearly  forty  miles  the  Southern  Pacific 
Railroad  has  erected  snowsheds  in  the  Sierra  Nevada  in  order 
to  maintain  traffic  during  the  winter  months.  The  depth  of 
the  snow  is,  moreover,  of  utmost  importance  to  many  power 
companies,  and  in  mining  and  irrigation  activities  also. 

60.  The  economic  importance  of  snow.  The  problem 
of  the  conservation  of  snow  and  its  dependence  upon  moun- 
tains and  forests  has  been  treated  in  detail  by  several  writers, 
and  more  especially  by  Professor  J.  E.  Church,  Jr.,  of  the 
University  of  Nevada.  The  water  content  of  the  snow, 
determined  by  weighing  with  a  snow  sampler  devised  by 
Church,  permits  quick  surveys  of  large  areas  of  snow,  both 
on  mountain  tops  and  in  gulches;  and  in  due  time  results  will 
be  forthcoming  which  will  enable  predictions  of  floods  or  in- 
sufficient water  supply,  as  well  as  studies  of  stream  flow  and 
water  resources.  The  author  has  discussed  the  relation  be- 
tween total  snowfall  and  depth  on  the  ground  in  characteristic 
an  effort  to  obtain  a  curve  characteristic  of  the  seasonal 
season.  Such  a  curve  would  in  effect  give  a  curve 
measure  of  the  heat  energy  expended  during  a  given  period  as 
determined  by  the  rate  of  disappearance  of  the  snow.  The 
disturbing  factor,  however,  is  wind  action ;  and  until  we  have 
some  record  of  the  direction  and  rate  of  flow  of  the  air,  and  also 
of  the  amount  of  water  vapor  carried  by  the  air  stream,  all 
forecasts  of  the  probable  water  supply  will  be  subject  to  error. 

The  Weather  Bureau  has  in  the  spring  months  tried  to 
determine  the  probable  amount  of  water  in  the  snow  cover 
of  the  high  levels,  available  for  irrigation.  The  plan  of  inten- 
sive surveys  in  small  watersheds  as  carried  out  by  Thiessen 
in  Utah  gives  approximate  information,  but  of  limited  char- 
acter. An  illustration  of  the  value  of  such  surveys  is  that 
made  in  the  watershed  of  City  Creek,  furnishing  water  to 

16 


226  THE  PRINCIPLES  OF  AEROGRAPHY 

Salt  Lake  City.  The  measurements  of  1915  indicated  a  prob- 
able water  yield  of  the  snow  cover  30  per  cent  less  than  that 
of  the  preceding  season,  and  also  that  the  snow  condition  was 
favorable  for  rapid  melting  and  early  run-off.  Similar  work 
has  been  attempted  in  the  watershed  of  the  Salt  River  of 
Arizona,  above  the  Roosevelt  Dam. 

From  the  records  made  at  Summit,  Placer  Co.,  California 
(elevation  2,138  meters),  covering  a  period  of  nearly  forty 
Forecasts  of  years,  it  will  be  seen  that  the  annual  snowfall  has 
water  supply  exceeded  19.68  meters  twice  in  a  period  of  thirty- 
five  years.  In  the  season  1879-1880  nearly  20 
meters  fell.  The  least  seasonal  snowfall  was  in  1880-1881,  the 
season  next  after  that  of  heaviest  snowfall.  The  total  amount 
was  less  than  4  meters.  The  average  depth  of  the  snow  is 
nearly  11  meters.  Records  of  the  depth  of  snow  on  the 
ground  have  been  kept  daily  since  1898. 

The  author  has  discussed1  the  method  used  by  Professor 
J.  N.  Le  Conte  in  1908  for  determining  the  mean  rate  of  melt- 
ing. It  was  found  that  the  curves  were  extremely  irregular 
for  the  eleven  seasons  considered  up  to  March  1,  but  fairly 
smooth  after  that  date.  Professor  Le  Conte  obtained  an 
average  curve  showing  the  mean  depth  of  snow  on  the  ground 
at  Summit.  The  curve  is  reproduced  in  Fig.  86.  In  many 
ways  the  period  July  1,  1910,  to  June  30,  1911, 
was  one  °^  t^ie  most  remarkable  on  record.  It 
followed  a  season  when  there  was  less  snow  in 
the  mountains  than  there  had  been  in  forty  years.  Up  to 
January  9,  1911,  the  season  was  exceptionally  dry  and  the 
snowfall  a  negligible  quantity.  Then  there  occurred  a  rapid 
change  to  the  other  extreme,  and  the  snow  fell  almost  steadily 
until  January  27,  when  there  was  approximately  5  meters 
on  the  ground.  February  was  a  month  of  moderate  snow- 
fall, also  of  moderate  melting.  The  total  decrease  was  only 
330  millimeters.  During  the  first  week  in  March,  2,286 
millimeters  of  snow  were  added.  Before  the  middle  of  the 
month  a  total  depth  of  7,740  millimeters  was  recorded.  Then 
followed  a  long  period  of  fair  weather,  permitting  a  rapid 
and  nearly  uniform  rate  of  melting,  the  depth  decreasing  at 

l  In  Monthly  Weather  Review,  June,  1910. 


PRECIPITATION 


227 


the  rate  of  about  203  millimeters  a  day.     It  was  noticeable, 
though,    that    the    melting    was    less    rapid    as    the    depth 


70 
50 

50 
40 
30 
20 
10 
0 

Solid 
depth  o 

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incnes 
300 

200 
100 

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FIG.  86.     DEPTH  OF  SNOW  AT  SUMMIT,  CAL. 

line  indicates  average  depth  of  snow,  mean  of  ten  seasons.     Dotte 
f  snow,  1910-1911.     A  BEFG,  average  rate  of  melting. 

decreased,  although  with  the  natural  increase  in  the  length 
of  the  day  and  the  approach  of  warmer  weather  the  contrary 
might  have  been  expected.  Without  doubt,  the  packing 
process  plays  an  important  part,  and  all  measurements  of 
depth  and  of  melting  must  be  corrected  for  this  factor.  The 
author  once  made  some  approximate  measurements  of  the 
water  content  of  snow  as  an  experiment  bearing  on  this 
problem.  Samples  of  snow  were  taken  from  the  top  and  the 
bottom  of  a  snow  bank  which  had  a  depth  of  about  3.6  meters. 
Melting  the  samples,  it  was  found  that  it  required  about  508 
millimeters  of  the  loosely  packed  snow  at  the  top  to  make 
30  millimeters  of  water,  while  of  the  more  compact,  almost 
slushy,  snow  at  the  bottom  it  required  only  102  millimeters 
to  make  25  millimeters  of  water. 

In  the  diagram  (Fig.  86)  the  dotted  lines  show  the  depth  of 
snow  on  the  ground  for  the  season  1910-1911.  The  solid  line 
represents  the  mean  depth  of  snow,  and  the  curve  marked 
ABEFG  is  the  mean  rate  of  melting  from  March  1  to  May 
26,  as  determined  by  Le  Conte. 

The  author  has  also  designed  a  means  of  comparing  the 
actual  curve  of  melting  for  any  given  season  with  the  mean 


228  THE  PRINCIPLES  OF  A&ROGRAPHY 


FIG.  87.     METHOD  OF  STUDYING  SNOW  COVER  IN  THE  MOUNTAINS 
AND  PROBABLE  RUN-OFF 

curve,  so  that  one  can  determine  the  probable  date  of  the 
disappearance  of  the  snow.  Fig.  87  represents  a  wooden  base 
Means  of  with  reinforced  pieces  at  suitable  intervals.  Small 
computing  grooves  are  cut  in  the  base  plate,  and  in  these 
:  melting  are  inserted  pieces  of  bristol  or  cardboard  cut  to 
represent  the  depth  of  the  snow.  As  there  is  practically  no 
snowfall  of  importance  during  July,  August,  and  September, 
the  year  begins  on  October  1.  March  1  falls  about  the  middle 
of  the  design,  and  we  thus  have  on  the  left  the  snowfall  of 
winter,  while  on  the  right  side  we  have  the  snowfall  of  the 
spring  months.  In  the  first  groove  there  is  inserted  a  card 
showing  the  mean  rate  of  melting.  This  can  be  slid  along  in 
the  groove  and  the  rate  compared  with  that  of  any  given  year 
by  bringing  into  line  the  two  profiles.  While  this  method 
cannot,  in  strictness,  be  said  to  give  the  true  rate  of  melting 
for  the  whole  year,  it  affords  an  approximate  measure  of  the 
rate  of  melting  under  normal  conditions.  Snowstorms  are 


PRECIPITATION  229 

shown  by  peaks  which  indicate  both  the  added  depth  and  the 
rate  of  melting  in  the  intervals  of  fair  weather.  At  the  right 
side  of  the  frame  vertical  strips  of  cardboard  show  the  total 
precipitation  for  the  season,  and,  by  means  of  suitable  notches, 
how  much  of  the  precipitation  was  snow.  The  whole  design 
enables  one  to  compare  readily  the  depth  of  snow  on  the  ground 
at  any  given  date  with  the  amount  during  previous  seasons. 

Summit  is  an  interesting  station  for  snowfall  work,  because 
86  per  cent  of  the  precipitation  falls  in  the  form  of  snow,  and 
most  of  the  rain  falls  in  July,  August,  and  Sep-  ^ind  an  im- 
tember,  practically  before  the  snow  cover  amounts  portant  factor 
to  anything.  It  is  true  that  occasionally  there  inn*elting 
will  come  a  warm  rain  in  January  or  February,  and  such  a 
condition  rapidly  reduces  the  depth  of  snow.  The  greatest 
factor,  however,  in  reducing  the  depth  is  probably  the  wind. 
Under  certain  conditions,  when  the  dry  air  moves  rapidly 
from  the  northeast,  decrease  by  evaporation  becomes  exces- 
sive. Two  or  three  such  days  will  lower  the  depth  200  milli- 
meters or  more. 

In  a  general  discussion  of  the  snowfall  of  the  eastern  United 
States,  C.  F.  Brooks  has  shown1  that  on  account  of  low  tem- 
perature and  dampness  the  Lake  region,  Appa-  Regions  of 
lachians,  and  North  Atlantic  coast  get  the  heaviest 
heaviest  snowfall.  The  Ohio  Valley,  South 
Atlantic  States,  and  Gulf  States  are  usually  too  warm  for 
much  snow.  In  the  Northwest  the  snowfall  is  moderate  because 
of  the  winter  dryness.  Within  the  larger  provinces  snowfall  is 
locally  modified  by  topography  and  exposure  to  moist  winds. 
Thus  the  Appalachians  get  heavier  snows  on  their  western  than 
on  their  eastern  slopes  (except  in  Vermont),  and  the  eastern 
shores  of  the  Great  Lakes  get  more  snow  than  the  western. 

Snow  generally  falls  in  connection  with  winter  cyclones, 

because  the  cyclonic  action  and  effect  of  topog- 

J       .    .        .  •     1        Greatest 

raphy  cause  precipitation.     The  northeast  wind       snowstorms 

is   the  wind   of   great   snowstorms.     The  north-       froij}1 
west  wind,  though  cold,  is  generally  dry  and  so 
brings  at  most  only  snow  flurries,  except  locally  on  a  windward 
mountain  slope  or  in  the  lee  of  the  Great  Lakes. 

1  In  Monthly  Weather  Review,  Jan.,  1915. 


230  THE  PRINCIPLES  OF  A&ROGRAPHY 

ICE    STORMS 

61.  The  various  types  of  ice  storms.  Bonacina  has  dis- 
cussed the  general  problem  of  frozen  precipitation  in  Symonss 
Meteorological  Magazine,  confirming  the  theory  of  Hellmann,1 

Dansey,  and  others  that  rain  falling  in  a  tern- 
frost"6  perature  below  the  freezing  point  produces  the 

phenomenon  of  " glazed  frost,"  or  "silver  thaw"; 
and  this  is  due  to  the  presence  of  a  warm  stratum  above  the 
cold  surface  air,  and  is  not  in  any  way  an  instance  of  super- 
cooling, or,  more  properly,  subcooling.  "In  England,"  Bona- 
cina says,  "a  warm  southerly  current  will  often  climb  over 
the  shoulders,  as  it  were,  of  a  cold,  easterly  surface  current, 
ultimately  replacing  it;  and  in  such  cases  the  premonitory 
symptom  of  a  thaw  is  liquid  rain  or  ice-rain,  falling  while  the 
Snow  hail  surface  temperature  is  still  well  below  the  freezing 
graupel,  sleet,  point.  In  this  manner  we  are  robbed  of  many 

an  expected  snowstorm.  The  cold  surface  air 
tends  to  freeze  the  rain,  and  so,  instead  of  liquid  rain,  we  get 
ice-rain.  And  this  leads  us  to  recognize  five  distinct  species 
of  frozen  precipitation.  These,  excluding  rime  or  hoarfrost, 
which  is  of  the  nature  of  deposition  rather  than  precipitation, 
are  snow,  hail,  graupel,  sleet,  and  ice-rain. 

1  An  elaborate  exposition  of  the  various  forms  of  water  vapor  is  given  by 
Hellmann  in  the  Konigl.  Preuss.  Met.  Inst.,  1915,  translated  by  C.  Abbe,  Jr.,  in 
the  Monthly  Weather  Review  for  July,  1916.  Hydrometeors,  in  the  narrow 
sense  of  the  word,  are  only  those  forms  of  condensation  that  bring  directly 
to  the  earth  water  in  its  liquid  or  solid  form,  so  that  the  clouds  (forming  a 
chapter  by  themselves)  are  not  included.  The  forms  are: 

1.  Direct  condensation  at  or  near  the  earth's  surface: 

Liquid  Solid 

"Sweat"  (Beschlag)  Frost  (Reif) 

Dew  (Tau)  Mist  ice  (Nebeleis) 

Mist  (Nebelwasser)  Ice  fog  (Eisnebel) 

Wet  fog  p .  f  Rauhreif  ) 

"Scotch "mist  (Nebelreissen)  (  Rauheis   j 

Fog  drip  (Nebeltraufe) 

Rain  without  clouds  Snow  without  clouds 

2.  Direct  condensation  in  the  free  air: 

Water  clouds  Ice  clouds 

3.  Indirect  condensation  in  the  free  air: 

Rain  (Regen)  Snow  (Schnee) 

Graupel       (Graupeln) 

Hail  (Hagel) 

Sleet  (Eiskorner) 

Glaze  or  glazed  frost  (Glatteis) 


PRECIPITATION  231 

' '  Snow  is  by  far  the  most  important  in  the  economy  of  nature 
and  is  produced  by  the  direct  passage  of  aqueous  vapor 
into  the  frozen  state;  it  is,  indeed,  next  to  rain,  the  most 
important  of  all  forms  of  atmospheric  precipitation.  Hail 
(hard  or  true  hail)  is  apparently  a  product  of  thunderstorm 
activity,  and  this,  together  with  its  peculiar  alternate  struc- 
ture, leads  one  to  suppose  that  the  freezing  raindrops  are 
carried  up  and  down  by  currents  many  times  before  they 
finally  strike  the  ground.  Graupel  (soft  hail)  is  the  little 
white  pellets  so  frequent  in  moderately  cold  weather;  as  it 
does  not  occur  in  severe  cold,  it  seems  reasonable  to  ascribe 
its  origin  to  the  passage  of  aqueous  vapor  first  into  liquid 
drops  which  freeze  before  falling.  Sleet  is  the  well-known 
mixture  of  raindrops  and  snowflakes.  Finally  we  have  the 
form  referred  to  above  as  being  associated  with  glazed  frost 
and  as  being  symptomatic  of  thaws.  It  takes  the  form  of 
plain  pellets  of  colorless  ice,  and  being  the  result  of  the  direct 
freezing  of  falling  raindrops  may  be  called  simply  ice-rain." 

62.  Air  temperature,  rain  temperature,  and  temperature 
of  objects.     C.  F.  Brooks  has  discussed  at  some  length   the 
ice  storms  of  New  England.     An  ice  storm  (verglas;  glatteis) 
occurs  when  raindrops  falling  on  trees  and  other        Conditions 
objects   cover   them   with   ice.     Using   the   Blue        producing 
Hill  records  for  various  air  levels,  Brooks  shows        verg  as 
that  there  are  a  number  of  combinations  of  different  condi- 
tions of  air  temperature,  rain  temperature,  and  temperature  of 
the  object  relative  to  freezing  which  may  produce  ice  storms. 
These  are: 

I.  Temperature  of  the  air  below  273°A. 
II.  Temperature  of  the  air  above  273° A. 

A.  Temperature  of  the  rain  below  273° A. : 

1.  From  passing  through  a  stratum  of  cold  air; 

2.  From  cooling  by  evaporation  in  non-saturated  air. 

B.  Temperature  of  the  rain  above  273° A. 
Temperature  of  the  object  below  273°A. : 

1.  From  residual  cold; 

2.  From  cooling  by  evaporation  in  non-saturated  air. 

As  is  readily  seen,  no  heavy  ice  storm  can  take  place  with 
the  surface-air  temperature  above  273°A.  In  fact,  no  con- 
siderable ice  storm  occurring  under  this  condition  has  been 


232  THE  PRINCIPLES  OF  AEROGRAPHY 

noted  at  Blue  Hill.  However,  from  theoretical  considerations 
they  are  possible.  Raindrops  may  be  cooled  far  below  273°A. 
Liquid  rain  without  solidifying.  It  is  well  known  that  fog 
below  the  particles  remain  in  the  liquid  state  at  tempera- 
freezing  point  tureg  far  below  273oA  The  lowegt  air  tempera_ 

ture  recorded  at  Blue  Hill  while  rain  was  falling  was  260°A. 
Undercooled  raindrops  freeze  almost  instantly  if  they  strike 
one  another  or  an  object.  Sometimes,  when  several  such 
Sizes  of  drops  come  together,  large  pieces  of  ice  may  be 

frozen  formed.  For  instance,  during  the  heavy  ice 

raindrops  gtorm  of  February  26,  1912,  in  Cambridge, 
Mass,  (air  temperature  about  271°A.),  the  diameters  of  the 
raindrops  averaged  about  0.5  millimeter;  but  the  smallest 
frozen  raindrops  were  1.5  millimeters  in  diameter  and  the 
largest  spherical  ones  4.5  millimeters,  while  some  rice-shaped 
pieces  of  ice  were  6  millimeters  long.1 

For  a  few  minutes  during  an  ice  storm  on  January  31, 
1914,  some  sleet  fell.  The  individual  pieces  were  generally 
spherical,  but  many  were  angular;  some  were  flat  on  one 
side,  and  others  were  agglomerations.  The  frozen  drops 
varied  between  0.5  millimeter  and  4  millimeters  in  diameter. 
The  structure  of  the  sleet  showed  plainly  the  general  con- 
ditions of  air  stratification.  Snowflakes  had  passed  into  an 
air  stratum  whose  temperature  was  above  273°A. ;  and  some 
of  them,  reaching  the  colder  layer  below  before  entirely  melt- 
ing, froze.  This  is  shown  by  the  presence  of  snowflake  skele- 
tons, angularity,  and  the  almost  invariable  presence  in  the  ice 
of  minute  bubbles.  The  unfrozen  rain  which  reached  the 
ground  was  rain  formed  within  the  warmer  stratum  and  of 
entirely  melted  snowflakes.  The  short  duration  of  the 
sleet  may  be  explained  by  assuming  that,  before  and  after 
its  occurrence,  the  warmer  layer  was  so  thick  that  none  of 
the  snow  passed  out  of  it  partially  melted. 

If  the  temperature  of  the  object  is  above  273°A.  the  ice 
will  partially  melt  and  fall  off  entirely,  or  else  be  frozen, 

1A  short  article  entitled  "The  Crystallization  of  Undercooled  Water,"  by 
Borus  Weinberg,  may  be  found  in  the  Monthly  Weather  Review,  1909,  pp.  14-15. 
See  also  W.  Meinardus  in  Das  Wetter,  Nov.,  1898,  p.  247,  the  ice-rain  of 
Oct.  20,  1898;  T.  Okada  in  Jour,  of  the  Met.  Soc.  of  Japan,  May,  1914;  W.  H. 
Dines  in  Symons's  Meteorological  Magazine,  Dec.,  1913;  E.  Gold,  ibid.,  Jan.,  1914. 


PRECIPITATION  233 

icicles  forming  from  melted  water.  When  the  temperature 
of  the  air  is  above  273°A.  the  ice  coating  will  be  continually 
melting,  but  no  icicles  will  form. 

When  the  temperature  of  the  raindrops  is  above  273°A.,  as 
it  generally  is,  a  smooth  coating  of  ice  forms;  also  icicles  from 
the  water  which  did  not  freeze  before  running  off.  The 
windmill  type  of  anemometer  then  becomes  a  fine,  glistening, 
star-shaped  affair.  A  temporary  ice  storm  may  occur  when 
both  the  rain  and  the  air  are  above  273°A.,  the  temperature 
of  the  object  being  below  that  point. 

The  ice  formed  in  ice  storms  is  generally  smooth,  but  it 
may  be  temporarily  rough  owing  to  an  admixture  of  snow 
or  sleet.1  Another  kind  of  roughness  may  develop  if  the 
water  freezes  slowly  after  it  has  fallen, —  ice  crystals  a  foot 
or  more  in  length  may  sometimes  be  seen  forming  on  a  cement 
sidewalk  during  an  ice  storm. 

Upper-air  conditions  of  temperature  now  require  attention 
after  the  foregoing  discussion  of  the  effect  produced.  To 
show  the  mode  of  formation  of  the  various  kinds  tipper-air 
of  precipitation,  the  accompanying  diagram  (Fig.  condition  in 
88)  has  been  constructed  from  Blue  Hill  observa- 
tions of  January  5-6,  1910.  It  shows  the  probable  conditions 
of  precipitation  and  temperature  from  a  height  of  1,300 
meters  to  the  ground  at  sea  level.  The  time  of  arrival  of 
each  stage  at  Blue  Hill  is  indicated  at  the  foot  of  the  diagram. 
Fig.  89  shows  the  results  due  to  the  passage  of  these  condi- 
tions at  the  summit  and  valley  stations.  From  Fig.  88  it 
may  be  seen  that  the  ice  storm  lasted  about  six  hours  at  the 
valley  station;  a  little  over  an  hour  at  195  meters  above  sea 
level  (summit),  and  that  a  station  above  400  meters'  altitude 
would  have  experienced  no  ice  storm  at  all.  Thus  local 
topography  has  a  great  effect  on  the  intensity  and  extent  of 
an  ice  storm. 

1  In  Blue  Hill  practice  "sleet"  is  any  frozen  form  of  falling  precipitation 
which  is  not  snow  or  summer  hail.  The  Weather  Bureau  has  decided  to 
restrict  the  term  "sleet "  for  official  purposes  to  the  small  particles  of  clear  ice 
which  frequently  fall  in  winter  with  or  without  an  admixture  of  rain.  The  term 
"rime"  is  used  for  a  coating  of  rough  ice  formed  on  terrestrial  objects  from  fog 
when  the  water  droplets  are  undercooled  so  that  they  turn  to  ice  on  coming  in 
contact  with  solid  bodies.  Rime  is  what  the  French  call  givre  and  the  Germans 
rauhreif. 


234 


THE  PRINCIPLES  OF  A&ROGRAPHY 


63.  Wind  conditions  which  produce  ice  storms.  What 
pressure  and  wind  distributions  cause  these  ice-storm  inver- 
sions of  temperature?  There  are  three  general  wind  condi- 
tions which  produce  ice  storms: 

1.  Warm  air  arriving  over  residual  cold  air. 

2.  Cold  air  coming  in  below  and  warm  air  arriving  above. 

3.  Cold  air  pushing  in  from  the  north  or  west  below  a  rain  cloud. 

The  ideal  conditions  for  the  first  are,  that  after  the  strong 
radiation  of  heat  from  the  lower-air  strata  to  the  ground 
in  an  anticyclone,  a  cyclone  rapidly  advances  toward  New 

England.  The 
storm  represented 
in  Figs.  88  and 
89  is  an  excellent 
illustration  of  this 
type  (which  will 
be  called  the 
"southerly" 
type).  Within 
twenty-four  hours 
a  strong  anticy- 
clone (1,043  kilo- 
bars)  over  New 
England  had  been 
replaced  by  a 
cyclone  from  the 
west-southwest, 
well  supplied  with  warm  moist  air  by  the  south  winds  in  front 
of  a  trough  of  low  pressure  extending  to  the  Gulf  of  Mexico.1 
The  record  in  Fig.  89  shows  a  regular  rise  in  temperature  at 
the  summit,  but  an  irregular  rise  in  the  valley.  Special  notice 
should  be  taken  of  the  change  between  3  A.  M.  and  4  A.  M. 
January  6,  at  the  lower  level.  This  clearly  illustrates  local 
air  drainage. 

The  ideal  conditions  for  the  second,  or  "northeasterly," 
type  are,  that  while  an  active  cyclone  in  the  south  is  supply- 
ing plenty  of  warm  air,  there  is  an  anticyclone  in  the  north 

1  For  a  theoretical  discussion  of  the  warm  south  wind  setting  in  over  cold 
stagnant  air,  see  W.  Schmidt,  "Weitere  Versuche  xiber  Boenvorgange  und 
das  Wegschaffen  der  Boden  Inversion,"  Meteorologische  Zeitschrift,  Sept.,  1913, 
pp.  446-447. 


2  3     OC. 

100  KILOMETRES 


FIG. 


Brooks 

CHART  OF  CONDITIONS  DURING  ICE  STORM 
JANUARY  5-6,   1910 


PRECIPITATION 


235 


giving  cold  air.  The 
northeast  wind 
blowing  toward  the 
southern  cyclone 
brings  in  the  cold  air 
from  the  anticy- 
clone, and  the  warm 
south  wind  of  the 
east  part  of  the  cy- 
clone brings  in  the 
warm  air  above. 
The  undercurrent 
of  cold  air  is  often 
not  cold  enough 
to  counteract  the 
warming  of  the 
southerly  wind,  and 
so  the  temperature 
may  rise  slowly  in- 
stead of  falling  or  re- 
maining stationary.1 
An  example  of 
this  northeasterly 
type  of  ice  storm  is 
that  of  February 
19-22,  1898.  Dur- 
ing this  time  the 
temperature  at  the 
top  of  the  hill  varied 
but  one  degree  (272° 
A.  to  273° A.);  the 
valley  temperature 
ranged  between 
274°A.  and  277°A. 
Thus  the  storm  was 
confined  for  the 
most  part  to  Blue 
Hill.  The  wind  was 
constantly  from  the 

1C.  F.  Brooks,  "Three  Ice  Storms,"  Science,  Aug.  8,  1913,  pp.  193-194. 


236  THE  PRINCIPLES  OF  A&ROGRAPHY 

northeast  and  its  velocity  was  5  to  15  meters  per  second;  69 
millimeters  of  rain  fell,  and  13  millimeters  more  of  sleet.  A  low- 
pressure  area  was  deadlocked  south  of  a  large  high. 

Another  storm  of  this  type  was  that  of  December  23,  1908. 
The  temperature  was  261°A.,  and  the  wind  north  by  east, 
10  meters  per  second.  This  ice  storm  occurred  during  a  short 
interval  (9:10-10:55  A.M.)  in  which  the  light  snow  changed  to 
rain.  An  anticyclone  was  settling  over  New  England  and  a 
cyclone  was  approaching  from  the  Gulf  of  Mexico. 

A  third   storm   of   this   type  is   worth   mentioning,   for   it 
occurred  during  a  kite  flight,  February  9,  1905.     Throughout 
the  kite  flight  the  wind  was  east-southeast,  but 
erly"  storm       the   temperature  was  gradually   falling    (272°A. 
during  t0  271°A.)-     The  valley  temperature  was  a  few 

degrees  higher.  Up  to  a  height  of  nearly  800 
meters  the  vertical  decrease  of  temperature  was  nearly  the 
normal  adiabatic;  but  at  885  meters  there  was  an  inversion 
to  273. 5°A.  from  a  minimum  of  270°A.  at  760  meters.  At 
that  level  was  the  base  of  an  arriving  warm  southeast  wind, 
all  the  precipitation  up  to  an  hour  and  a  half  previously 
having  been  snow.  On  account  of  ice  accumulations  on  the 
lower  kites  and  wire,  a  kite  flight  during  an  ice  storm  is  never 
long  continued,  nor  are  high  altitudes  reached. 

A  storm  of  this  type,  which  occurred  over  all  north  Ger- 
many, October  19-21,  1898,  is  fully  described1  by  Meinardus. 
This  storm  was  well  observed  as  to  conditions  up 
te  stoSTow  to  a  heiSht  of  2>500  meters.  On  the  morning  of 
the  same  area  the  20th,  in  the  lower  levels,  a  poorly  developed 
fevels 6rent  cyclone  was  centered  in  southwest  Germany,  caus- 
ing a  northeast  indraft  of  cold  air  from  an  anti- 
cylone  over  west  Russia.  At  an  altitude  of  2,500  meters  the 
center  of  the  storm  was  over  central  Germany,  and  it  was  well 
developed.  At  that  altitude,  it  caused  over  eastern  Germany 
a  warm  south  wind,  six  to  nine  degrees  warmer  than  the 
northeast  wind  below.  Over  western  Germany  there  was 
a  cold  northwest  wind  over  the  warmer  northeast  wind.  A 
mountain  station,  1,600  meters  high,  experienced  this  and 
had  an  ice  storm  from  the  rain  that  fell  from  a  warm  layer 

iDas  Wetter,  Nov.,  1898,  pp.  247-260. 


PRECIPITATION  237 

above  it,  and  there  was  also  an  ice  storm  below  from  the  still 
unfrozen  rain  that  fell  through  this  very  cold  wind.  Thus 
there  were  both  the  northeasterly  and  northwesterly  types 
of  ice  storms  in  progress  at  the  same  time  at  different  levels. 

This  third  or  "  northwesterly "  type  is  about  the  reverse  of 
the  first.  Here  the  cold  air  apparently  enters  like  a  wedge 
below,  while  it  is  still  raining  above.  The  The  «north. 
changes  in  the  form  of  the  precipitation  which  westerly" 
occur  are  in  the  reverse  order  of  those  of  the  type  of  storm 
southerly  type.  This  process  of  change  from  rain  to  snow 
can  be  seen  easily  by  moving  the  conditions  represented  in 
Fig.  88  from  left  to  right  over  an  imaginary  piece  of  country. 
Under  such  circumstances  a  wedge  of  cold  air  from  the  north- 
west is  represented  entering  below  a  warmer  southerly  or 
northeasterly  wind.  The  boundary  between  these  two 
masses  of  air  is  known  as  the  wind-shift  line.  The  passage 
of  a  wind-shift  line  is  a  common  phenomenon,  but  it  is  only 
occasionally  that  conditions  of  temperature  and  rainfall  are 
such  as  to  make  an  ice  storm. 

A  representative  storm  of  this  type  occurred  February  15, 
1906.     At  2  A.M.,  with  a  northeast  wind  and  rain,  the  ice 
storm  (type  two  at  the  start)  began,  when  the    Exampie  Of 
temperature  fell  to  273°A.     The  temperature  in    northwesterly 
the  valley  began  to  fall  rapidly  at   2:10,  and  on    type 
the  hill  fifteen  minutes  later,  the  wind  shifting  to  the  north 
and  increasing  from  9  to  11  meters  per  second.     Snow  began, 
at   first   mixed   with   rain.     When   the  rain  ceased  the  snow 
continued  to   3:45,  the  temperature  having  reached  266°A. 
at  Blue  Hill. 

Owing  to  the  fact  that  often  a  single  ice  storm  exhibits  the 
characteristics  of  two  or  even  three  of  the  types  described 
above,   it  is  necessary  to   classify  them  accord-    A11  ice  storms 
ing    to    the    positions    and    movements    of    the    of  two  large 
cyclones    and   anticyclones   which   produced   the    c  ai 
ice-storm   conditions.     Thus  two  large  divisions  have   been 
made,    more    or    less    arbitrarily.     The    first    includes    those 
storms  in  which   there  were  anticyclones  in  the  north  domi- 
nating southern  cyclones;  the  second  includes  those  in  which 
the  cyclones  and  anticyclones  were  in  regular  sequence. 


238  THE  PRINCIPLES  OF  AEROGRAPHY 

It  is  noteworthy  that  all  the  ice  storms  occurring  when  a 
northern  high  dominated  a  southern  low  were  of  either  type 
two  or  of  types  two  and  three  combined.  Eleven  out  of 
the  thirty-one  occurring  under  these  conditions  were  severe. 

Most  of  the  ice  storms  occur  when  the  cyclones  follow 
the  anticyclones  from  the  west  or  southwest,  severe  storms 
being  most  common  when  the  cyclone  comes  from  the  south- 
west (Gulf  of  Mexico).  In  greatest  frequency  come  the  ice 
storms  of  the  northeasterly  type,  occurring  in  116  instances. 
Next  is  the  southerly  type,  occurring  67  times,  and  last  is 
the  northwesterly  type,  occurring  59  times.  The  northeasterly 
type  is  'favored  by  southern  lows  and  northern  highs;  the 
southerly  type  by  the  low  crowding  in  close  behind  the  high, 
and  the  northwesterly  type  coming  most  frequently  when 
the  high  arrives  close  behind  the  low. 

In  the  distribution  by  months  in  which  ice  storms  occur, 
48  came  in  January,  46  in  February,  40  in  March,  27  in  De- 
Distribution  cember,  10  in  November,  and  7  in  April.  The 
of  ice  storms  average  is  12  a  year.  The  earliest  ice  storm  in 
by  months  the  faU  came  November  8_10>  1894>  and  the 

latest  in  the  spring,  April  30,  1909.1 

A  study  of  some  of  the  details  of  the  ice  storms  shows  that 
they  may  occur  with  a  temperature  as  low  as  260°A. ;  that  it 

may  rain  hard  or  lightly;  that  the  wind  may 
Various  combi-         J  °       J>  J 

nations  of         come  from  any  direction  and  blow  a  gale  or  not 

conditions  in  at  a\\.  that  the  temperature  may  rise,  fall,  or 
ice  storms  .  ^  ,  J 

remain  stationary;  and  that  through  the  various 

combinations  of  these  conditions,  a  single  ice  storm  may 
change  through  all  the  types  and  then  back  again  or  stay 
consistently  of  a  single  type.  In  fact,  as  has  been  stated, 
provided  it  is  raining  and  the  air  temperature  is  below  freez- 
ing, an  ice  storm  can  occur  under  practically  any  consistent 
combination  of  meteorological  conditions. 

A  few  extraordinary  features  occurring  during  some  ice 
storms  not  already  mentioned  may  here  be  described.  Dur- 
ing the  storm  of  January  22-23,  1904,  the  sudden  changes 
in  wind  and  temperature  were  the  most  remarkable  ever 

1  The  unpublished  details  of  the  178  ice  storms  to  March  25,  1914,  have 
been  tabulated  and  placed  in  the  library  of  the  Blue  Hill  Observatory. 


PRECIPITATION 


239 


recorded  at  Blue  Hill 
Ob  servat  ory .  Fig. 
90  is  a  tracing,  for 
comparison,  of  the 
thermograph  curves 
of  the  valley  and 
summit  stations  on 
the  same  sheet;  it 
shows  a  marked  in- 
version of  tempera- 
ture, and  also  the 
variations  in  tem- 
perature due  to 
warm  south  winds 
at  the  upper  level, 
which  did  not  affect 
the  lower  stratum. 
It  also  illustrates  the 
relation  of  gusty 
winds  and  tempera- 
ture. There  is  an 
extraordinary  inver- 
sion of  8  degrees 
indicated,  which 
lasted  many  hours. 
The  sudden  alter- 
nating gusts  of 
wind  seemed  to  af- 
fect the  top  of  the 
hill  only,  and  made 
the  temperature  go 
up  and  down  5  de- 
grees or  more,  in 
almost  as  many 
seconds.  The  sum- 
mit of  Blue  Hill 
was  evidently  near 
the  level  of  the 
bounding  surface 


240 


THE  PRINCIPLES  OF  AEROGRAPHY 


between  a  warm  southerly  current  above  and  a  cold  north- 
erly one  below.  While  the  hill  was  in  the  upper  current 

the  temperature 
was  high,  but  while 
the  dividing  sur- 
face remained 
above  the  hilltop 
the  summit  was 
swept  by  a  cold 
north  wind.  In 
this  case  the  line 
was  so  sharp,  or  its 
vertical  movement 
so  rapid,  that  the 
change  from  one 
current  to  the  other 
was  almost  instan- 
taneous. The  ini- 
tial cold  indraft  was 
rather  significant, 

FIG.  91.     ICE  STORM,  JANUARY  18,  1909  *°r      1^      made      the 

temperature  at  the 

valley  station  fall  more  than  an  hour  before  the  tempera- 
ture on  the  hill  was  in  the  least  affected.  These  conditions 
show  how  slowly  a  warm  current  of  air  affects  a  cold  lower 
stratum. 

Figs.  91-94  illustrate  the  ice  formation  during  such  storms 
as  have  been  described,  from  photographs  by  L.  A.  Wells. 

Okada  has  studied  the  rate  of  cooling  of  drops  of  rain 
falling  through  a  cold  layer.  Since  the  rate  of  evaporation 
Rate  of  *s  tne  same  as  it  is  a^  the  surface,  and  the  radius 

cooling  of          decreases  at  a  constant  rate,  which  rate  he  found 
rops          to  be  0.000033  millimeter  per  second,  the  rate  of 
evaporation  was 

q  =  3 . 3  X  10~6  X  I2  mm.  per  second. 

Assuming  the  temperature  of  the  air  stratum  through  which 
the  drop  falls  as  271. 5°A.,  and  the  diameter  of  the  drop  as 
2  millimeters,  and  the  rate  of  fall  6  meters  per  second,  then 
it  would  take  the  drop  about  145  seconds  to  fall  through  the 


PRECIPITATION 


241 


stratum  of  870  meters,  which  Okada  found  was  the  height 

at  which  strong  winds  from  the  southwest  were 

blowing   and  rain  was  falling.     But  the  rate  of 

evaporation  is  proportional  to  the  saturation  def- 

icit of  the  air,  in  this  case  0.4  millimeter,  so  that  the  rate 

for  the  water  drop  was 

<?x°'4 
X3To 

Again,  the  rate  of  evaporation  increases  as  the  square  root 
of  the  wind  movement.  Assume  an  air  movement  of  0.1 
millimeter  per  second,  then  the  rate  of  evaporation  would  be 


therefore 

t  =  145  seconds  ; 
2=l.lXlO-6mm/s; 
p  (density)  =  1  ; 
specific  heat  c  =  1  ; 

radius  r  =  0  .  1  cm.  ; 
/  (latent  heat  of  vaporization)  =  600  gram  calories. 

The  final  tem- 
perature of  the 
drop  will  b  e 
270°A.,  from 
which  it  appears 
that  raindrops 
falling  through 
ice-cold  layers 
may  be  suffi- 
ciently cooled  by 
evaporation  and 
conduction  to 
below  the  freez- 
ing point  and  so 
cover  the  objects 
on  which  they  fall 
with  a  coating  of 
ice.  In  some 

.1          •  FIG.  92.     ICE  STORM,  BLUE  HILL,  JANUARY  18,  1909 

cases  the  air  may 


17 


242 


THE  PRINCIPLES  OF  A&ROGRAPHY 


be  so  moist  that  the  drops  would  cool  only  to  the  dew  point 
after  falling  a  few  meters  from  the  mother  cloud.     In  that 

case,  condensation, 
instead  of  evapora- 
tion, would  begin 
on  the  drop  surface 
and  there  could  be 
no  glazed  frost. 

The  theory  of 
the  formation  of 
hail  is  given  in  the 
chapter  on  thunder 
storms  (pp.  184- 
188).  Many  reli- 
able records  of 
hailstones  weighing 
more  than  150 
grams  can  be  found 
in  Rollo  Russell's 
book  Hail,  pub- 
lished in  1893.  For 
many  years  it  has 
been  the  practice  in  certain  parts  of  Europe  to  attempt 
to  prevent  the  formation  of  hail  by  shooting  vortex  smoke 
rings  from  cannon  of  a  certain  make.  Despite  general 
assurances  made  by  those  interested  in  the  sale  of  such 
dissipators,  careful  and  disinterested  investigation  fails  to 
support  the  contention  that  such  means  are  of  any  real 
value.  They  are  in  the  same  class  with  the  attempts  made 
in  the  United  States,  Australia,  and  elsewhere  to  produce 
rain  by  bombardment  or  to  facilitate  condensation  and  pre- 
cipitation by  explosions. 

DEW,    HOARFROST,    GIVRE 

64.  Formation  of  dew  and  frost.  Dew  (rosee)  (tau)  and 
hoarfrost  (gelee  blanche)  (reif)  differ  in  their  manner  of  for- 
mation solely  in  the  conditions  of  temperature  under  which 
they  are  produced. 

Dew   is  the   name    given   to  the    drops    of    liquid    water 


FIG.  93.  ICE  STORM,  FEBRUARY  18,  1910 


PRECIPITATION 


243 


condensed  on  objects  from  the  atmosphere.     The  precipitation 

is  due  to  the  cooling  of  the  objects  by  radiation  below  the 

dew  point    of    the   atmosphere.      It    is    for   this 

reason    that    dew  is    most    frequently    observed        Of°dewtl0n 

on    fine   evenings   or  nights,   on    the    horizontal 

surfaces  of  objects  possessing  small  capacity  and  conductivity 

for  heat    (provided   they  are  insulated  from   conduction   of 

heat  from  below).1 

If  saturation  is  reached  under  similar  conditions,  but  at 
temperatures  below  the  freezing  point,  hoarfrost  is  formed; 
that  is  to  say,  minute  needle-shaped  crystals  of 
white  ice  appear  on  the  exposed  surfaces,  giving  hoarfrost"  °f 
them  a  dull  silvery  appearance.  Hoarfrost  is 
not  always  frozen  dew.  If  the  dew  point  is  273°A.  it  may 
happen  that  dew  and  hoarfrost  will  form  side  by  side  accord- 
ing to  the  radiative  power  of  the  object.  In  such  conditions 
the  hoarfrost  is  more  copious  because  the  vapor  pressure 
over  ice  is  lower  than  over  water  and  therefore  condensation 
is  more  rapid. 

From  the  con- 
ditions attending 
their  formation, 
both  dew  and 
hoarfrost  occur 
with  falling  tem- 
perature. 

"Givre"(rime) 
is  a  form  of  frost 
which  occurs  in 
nature  as  the  re- 
sult of  two  dis- 
tinct processes. 
In  view  of  this 
fact,  and  because 
that  which  is  pre- 
cipitated  is  in 
each  case  unlike 


FIG.  94.     ICE  STORM,  JANUARY  15,  1912 


Wells 


a  remarkable  collection  of  illustrations  of  frost  crystals,  showing 
especially  hoarfrost  crystals,  see  Wilson  A.  Bentley  in  Monthly  Weather  Review, 
Aug.,  Sept.,  and  Oct.,  1907. 


244  THE  PRINCIPLES  OF  ARROGRAPHY 

in  external  appearance,  the  two  phenomena  should  be  dis- 
tinguished as  follows: 

There  are  two  kinds  of  givre. 

1.  Givre    which    condenses    on     the     objects     themselves. 
It  has  the  appearance  of  needle-shaped  crystals  of  ice  and 
Givre  which      forms  when  frost  gives  way  suddenly  to  warm 
condenses  on    and  moist  weather,  on  objects  of  which  the  tem- 
perature is  still  below  the  freezing  point.     These 

objects  must  have  great  capacity  and  small  conductivity  for 
heat.  Givre  of  this  kind  closely  resembles  hoarfrost  in  exter- 
nal appearance;  but  it  forms,  as  a  rule,  with  an  overcast  sky 
and  always  during  a  rise  of  temperature. 

2.  Givre  deposited  from  the  air.     This   is  a  deposit  from 
the  air  of  subcooled  droplets  of  water,  which  congeal  as  soon 
Givre  de-          as    they    come    in    contact    with    solid    objects, 
posited  from      Givre  of   this  kind  is   deposited  most   copiously 

on  the  side  of  objects  exposed  to  the  wind.  It 
forms  a  semi-transparent,  rough  covering  of  ice. 

The  English  name  for  the  coating  of  ice  so  generally  known 
as  "sleet"  but  erroneously  so,  is  glazed  frost,  verglas  in  French, 
and  glatteis  in  German.  Perhaps  the  most  satisfactory  name 
for  the  coating  of  ice  which  occurs  when  cold  rain  falls  on 
colder  surfaces  is  glaze. 

65.  Dew  deposits.  In  1814  W.  C.  Wells  published  his 
now  well-known  theory  of  dew;  and  while  there  had  been  con- 
siderable experimentation  on  the  subject  before  this  treatise, 
like  Howard's  classification  of  the  clouds  it  was  so  generally 
accepted  that  comparatively  little  experimentation  followed. 
However,  there  is  room  for  further  investigation,  particularly 
in  connection  with  accurate  determination  of  nocturnal 
radiation.  Dew  measurements  have  been  practically  over- 
looked by  official  weather  services,  and  one  searches  in  vain 
for  records  of  the  daily  amounts.  Dines1  has  shown  that 
Annual  tne  general  belief  that  the  annual  dew  deposit 

deposit  of  in  England  amounted  to  about  120  millimeters 
(as  given  in  a  footnote  in  the  latest  edition  of 
Wells'  Essay  on  Dew)  is  an  overestimate.  He  estimates  the 
average  annual  deposit  of  dew  upon  the  surface  of  the  earth  to 

1  Quart.  Jour,  of  the  Royal  Met.  Soc.,  July,  1873,  p.  157. 


PRECIPITATION  245 

be  less  than  37  millimeters.  It  is  not  an  easy  matter  to 
record  dew.  Results  cannot  be  obtained  on  nights  when 
rain  is  falling,  nor  when  there  is  much  wind  and  rapid  evapo- 
ration. And  the  observer  must  be  up  before  sunrise  at  all 
seasons;  that  is,  before  rapid  evaporation  begins.  Dew  can- 
not be  measured,  like  rainfall,  at  given  hours.  Dines  found 
that  only  on  rare  occasions  did  the  amount  of  dew  exceed 
0.25  millimeter.  Out  of  198  observations  this  amount  was 
exceeded  only  three  times.  The  average  amount  was  about 
.001  millimeter.  These  amounts  were  determined  by  weighing. 
The  drosometer,  or  dew  measurer,  is  an  instrument  devised  by 
S.  Skinner.  It  consists  of  a  hemispherical  vacuum  Measurement 
glass  vessel,  jacketed,  of  the  .type  designed  by  of  dew 
Dewar  for  holding  liquid  air.  The  cup  has  a  deP°slt 
diameter  of  11.2  centimeters,  exposing  a  virtual  surface  aperture 
of  98  square  centimeters.  The  vacuum  is  a  good  nonconductor, 
and  the  heat  lost  by  radiation  from  the  inner  surface  of  the 
cup  must  be  supplied  from  the  air  in  the  cup;  as  soon  as  this 
falls  to  the  dew  point,  moisture  is  deposited.  From  measure- 
ments made  on  34  nights,  Skinner  is  of  opinion  that  the 
annual  dew  deposit  is  about  2.33  centimeters  a  year.  Skinner 
also  discusses  the  value  of  the  rain  gauge  as  a  dew  collector. 
There  were  seven  nights,  for  example,  on  which  the  dew 
exceeded  2.5  millimeters.  But  there  was  no  trace  of  rain  in 
the  rain  gauge,  which  was  of  the  old  Howard  pattern,  that 
is,  125-millimeter  copper  funnel  standing  in  a  bottle.  The 
dew  had  evidently  formed  on  the  outer  or  under  side  of  the 
metal  funnel  and  run  down  outside.  In  general,  dew  forms 
on  the  upper  side  of  a  blade  of  grass  or  in  a  large  drop  at  the 
tip  of  the  blade. 


CHAPTER  XVI 

FLOODS  AND  NOTABLE  STORMS 

66.  The   relation   between    storm   frequency   and  floods. 

Unusually  heavy  rains  are  nearly  always  followed  by  heavy 
run-off  and  floods.  In  some  countries,  as  for  example  China, 
there  is  a  direct  relation  between  the  duration  of  the  winds 
from  the  Pacific,  and  floods.  In  the  United  States  the  precip- 
itation that  directly  causes  floods  in  the  Mississippi  can  be 
traced  to  storms  moving  from  the  southwest.  One  of  the 
important  functions  of  the  Weather  Bureau  is  the  forecasting 

of  such  conditions  and  the  issuing  of  warnings. 
floods*8  mg  These  warnings  may  anticipate  flood  conditions 

by  periods  ranging  from  three  days  to  four 
weeks.  It  is  stated  by  Frankenfield  in  his  Report  on  the 
Floods  of  the  Ohio  and  Mississippi,  in  1912,  that  the  variations 
of  the  actual  from  the  forecast  stages  in  all  except  the  pre- 
cipitous mountain  streams  were  practically  negligible.  For 

example,  the  warnings  for  New  Orleans  issued  by 
floodrwayrnhigs  the  forecaster  (Dr.  I.  M.  Cline)  nearly  five  weeks 

in  advance  were  not  materially  changed  except 
as  to  the  date  of  occurrence  of  the  crest  stage,  since  numerous 
crevasses  at  times  interfered  with  the  expected  crest  progress. 
The  great  drainage  basin  of  the  Mississippi  River  covers 
an  area  of  about  3,211,729  square  kilometers,  or  1,240,050 
square  miles,  about  41  per  cent  of  the  total  area  of  the  United 
States,  Alaska  excluded.  There  are  six  distinguishable  grand 
basins,  five  of  them  watersheds  of  the  principal  large  rivers, 
namely,  the  Missouri,  the  Ohio,  the  Arkansas,  the  upper 
Mississippi,  and  the  Red  (see  chart  showing  drainage  basin 
of  the  Mississippi,  Fig.  95.) 

Notable  floods  have  occurred  in  the  lower  Mississippi  in 
1815,  1828,  1844,  1849,  1850,  1851,  1858,  1859,  1862,  1865, 
1881,  1883,  1892,  1903,  1909,  and  1912,  the  last  being  the 
greatest  flood  on  record.  A  severe  storm  from  the  southwest 
moved  over  the  Ohio  valley  on  February  21,  1912,  succeeded 

246 


FLOODS  AND  NOTABLE  STORMS 


247 


248  THE  PRINCIPLES  OF  A&ROGRAPHY 

by  another  storm  of  similar  character  four  days  later.     During 
the    first    decade   of   March   storms   were  frequent   but    not 

heavy.  On  March  10,  an  extensive  depression 
of  1912°  moved  in  from  the  Pacific  to  southern  California, 

and  by  the  following  day  had  reached  Kansas. 
The  rain  and  snow  were  moderately  heavy.  While  this  storm 
was  moving  across  the  country,  another  disturbance  appeared 
on  the  North  Pacific  coast,  and  by  the  time  (March  14)  the 
first  storm  had  passed  to  Newfoundland  the  second  storm 
was  over  Kansas,  accompanied,  like  its  predecessor,  by  a 
secondary  depression  extending  southward  over  southeastern 
Texas.  This  second  storm  passed  into  the  North  Atlantic 
Ocean  during  the  night  of  March  15-16,  and  for  four  days 
there  was  no  precipitation  of  consequence.  On  March  19  a 
depression  was  noted  over  Utah,  following  one  over  Kansas. 
The  depression  in  the  Great  Basin  was  probably  only  a  further 
development  of  the  marked  depression  on  the  North  Pacific 
coast  of  March  15-16,  a  fact  which  appears  to  have  been 
overlooked  by  the  writers  on  the  floods  of  March,  1912.  This 
storm  passed  into  the  North  Atlantic  during  the  night  of 
March  21-22.  On  March  23  a  well-marked  storm,  appeared 
over  the  west  Gulf,  reached  the  Ohio  valley  on  the  24th,  and, 
passing  rapidly  to  southern  New  England,  moved  thence  to 
sea.  It  would  seem  then  that  there  is  a  definite  relation 
between  storm  frequency  and  floods. 

The  flood  of  March-April,  1913,  which  began  on  March  23, 
was  caused  solely  by  excessive  precipitation  over  a  large  area. 

This  heavy  rainfall  caused  the  rivers  of  northern 
of  1913°  Indiana  and  Ohio,  especially  the  Miami,  Scioto, 

and  Muskingum,  to  rise  rapidly.  Only  a  small 
part  of  the  great  damage  done  can  be  attributed  to  the  break- 
ing of  dams.  These  northern  tributaries  of  the  Ohio  are  not 
as  a  rule  operative  in  causing  floods  in  the  Ohio.  In  this 
instance  it  happened,  however,  that  the  eastern  and  southern 
tributaries  were  also  carrying  large  volumes  of  water.  In  an 
official  report  on  this  flood1  it  is  stated  that  the  almost 
inconceivably  extensive  damage  done  was  increased  by 
the  work  of  man  in  the  channels,  along  the  banks,  and 

1  Horton  and  Jackson,  Water  Supply  Paper  No.  334,  U.  S.  Geological  Survey. 


FLOODS  AND  NOTABLE  STORMS 


249 


250 


THE  PRINCIPLES  OF  A&ROGRAPHY 


From  Water  Supply  Paper  334,  U.  S    Geol.  Surv. 

FIG.  97.     MIAMI  STREET  CANAL  BRIDGE,  DAYTON,  OHIO,  AFTER  THE  FLOOD 
OF  MARCH-APRIL,   1913 

across  the  river  valleys.  The  ground  was  not  frozen;  but 
it  was  practically  saturated  by  previous  rains,  and  so  did 
not  permit  the  storage  of  any  considerable  amount  of 
water.  It  is  doubtful,  however,  if  ground  storage  even 
under  the  most  favorable  conditions  would  have  had  any 
material  effect,  because  of  the  intensity  of  the  precipi- 
tation. In  the  five  days  from  March  23  to  27  the  rainfall 
averaged  from  100  to  250  millimeters.  In  advance  of  the 
first  storm,  which  caused  the  tornadoes  of  the  23d,  a  marked 
hyperbar  (area  of  permanent  high  pressure)  drifted  slowly 
across  the  United  States,  settling  over  the  Bermudas  on  the 
27th.  Thus  while  a  second  storm  was  trying  to  move  east- 
ward during  the  24th,  there  was  an  area  of  high  pressure 
off  the  Atlantic  coast,  and  another  spreading  eastward  from 
The  most  ^e  re^on  °f  tne  Great  Lakes.  At  8  P.M.  of  the 
disastrous  24th  these  two  areas  were  separated  only  by  a 

Ohid  °alihe       ^ane  °^  ^ow  Pressure  °ver  the  Ohio  basin,   con- 
necting an  approaching  and  a  departing   storm. 
Thus  the  rain  areas  of  two  storms  came  together  and  caused 
the  most  disastrous  flood  in  the  history  of  the  Ohio  valley. 


FLOODS  AND  NOTABLE  STORMS 


251 


252 


THE  PRINCIPLES  OF  A&ROGRAPHY 


From  Water  Supply  Paper  334,  U.  S.  Geol.  Surv. 

FIG.  99.     POST-OFFICE,  DAYTON,  OHIO,  AFTER  -THE  FLOOD  OF 
MARCH-APRIL,   1913 

(Figs.  97  and  99.)  No  extremely  low  temperatures  occurred 
during  the  flood;  the  ground  was  not  frozen,  and  there  was 
no  snow  or  ice  of  any  consequence  stored  in  the  drainage 
basin.  The  total  damage,  as  estimated  in  the  report  of 
the  engineers,  was  $180,000,000.  The  conditions  at  Dayton, 
Middletown,  Hamilton, 'Piqua,  Zanesville,  and  other  localities 
were  beyond  description. 

67.  The  Galveston  storms.  Perhaps  the  most  destructive 
single  storm  of  modern  times  occurred  on  the  night  of  Septem- 
ber 8,  1900,  when  a  West  Indian  hurricane  passed  over 
Galveston.  In  that  locality  alone,  more  than  6,000  persons 
lost  their  lives  through  drowning  or  injury  from  falling 
buildings  and  storm  damage.  Property  worth  $30,000,000 
was  destroyed  within  the  city  limits  and  an  enormous  amount 
of  property  was  ruined,  in  the  interior  and  along  the  coast, 
with  considerable  loss  of  life.  At  Galveston  a  pressure  reading 
as  low  as  964  kilobars  (28.48  inches)  was  recorded,  which  was 
lower  by  3  kilobars  (.10  inch)  than  had  been  previously  reported 
from  any  station  in  the  United  States.  The  highest  wind 


FLOODS  AND  NOTABLE  STORMS 


253 


velocity  was  38  meters  per  second  (84  miles  per  hour)  at 
6: 15  P.M.,  and  2  miles  were  registered  at  the  rate  of  45  meters 
per  second  (100  miles  per  hour).  But  at  this  time  the  ane- 
mometer was  blown  away  and  an  estimate  of  the  velocity  of 
50  m.p.s.  (112  m.p.h.)  seems  not  unreasonable.  The  devas- 
tation at  Galveston  was  in  large  measure  due  to  a  wave  which 
swept  in  from  the  Gulf  in  advance  of  the  vortex  of  the  storm. 
A  detailed  description  of  the  hurricane  is  given  by  Cline  in 
the  Monthly  Weather  Review  for  September,  1900,  p.  372. 
For  aq.  interval  of  nearly  fifteen  years,  with  the  exception  of 
a  storm  of  moderate  violence  on  July  21,  1909,  this  part  of 
the  coast  of  Texas  was  free  from  hurricanes.  On  August  16, 
1915,  a  marked  vortex  approached  the  east  Texas  coast  and 
by  8  P.M.  of  the  next  day  the  pressure  at  Galveston  was  down 
to  985  kilobars  with  high  northeast  winds.  During  the  night 

Mi  INCHES 
10JM30.5 


30.0 


29-5 


29.0 


28.5 


8  10  XII2   4    6   8  KTM'2   4 


950 


29.0 


FIG.  100. 


BAROMETRIC  PRESSURE,  HOUSTON,  TEXAS,  DURING  THE 
STORM  OF  AUGUST  16-17,  1915 


of  August  16-17  the  storm  passed  slowly  northwestward;  and 
at  5:30  A.M.  of  the  17th  the  pressure  at  Houston  (Fig.  100) 
was  down  to  955  kilobars1  (28.20  inches),  the  lowest  reading 

1  See  p.  258,  where  a  reading  ot  952  kbs.  is  given. 


254 


THE  PRINCIPLES  OF  AEROGRAPHY 


FIG.  101.     PATHS  OF  THE  GALVESTON  HURRICANES  OF  1900  AND  1915 

of  the  storm.  Within  a  few  hours  the  storm  recurved  to  the 
northeast,  moving  slowly  and  accompanied  by  torrential  rains 
and  high  winds.  A  comparison  of  the  paths  of  the  storms  of 
1900  and  1915  has  been  given  by  Frankenfield  in  a  special 
bulletin  from  which  the  following  extract  is  taken.  (See 
Fig.  101,  which  shows  the  paths  of  the  Galveston  hurricanes 
of  1900  and  1915.) 

"An  inspection  of  these  paths  discloses  the  fact  that  the 
total  time  occupied  from  the  first  to  the  last  appearance  of 
both  storms  within  the  field  of  observation  was  exactly  four- 
teen days,  and  that  the  storm  of  1900  moved  with  a  slower 
velocity  of  progression  before  reaching  its  recurve  than  after, 


FLOODS  AND  NOTABLE  STORMS 


255 


whereas  in  the  storm 
of  1915  the  reverse  was 
true.  The  two  paths 
are  very  similar  in  many 
respects,  although  that 
of  1915  lay  a  little  to 
the  southward  of  that 
of  1900  until  the  St. 
Lawrence  Valley  was 
reached.  In  previous 
published  reports  on 
the  storm  of  1900  the 
storm  path  shows  a 
strong  deflection  toward 
the  southwest  Florida 
coast,  but  reports  re- 
ceived from  vessels  and 
other  sources  after  those 
publications  indicated 
the  fact  that  this  deflec- 
tion to  the  right  was 
not  so  strong  as  has 
been  supposed,  and  the 
track  as  here  charted 
is  thought  to  represent 
more  nearly  the  true 
conditions.  It  was 
carefully  plotted  from 
all  available  observa- 
tions. As  to  the  com- 
parative intensities  of 
the  two  storms,  it  is  per- 
haps idle  to  speculate. 
The  wind  velocities  were 
not  greatly  different, 
and  the  effects  of  the 
two  storms  were  much 
the  same,  except  as 
modified  by  artificial 


256 


THE  PRINCIPLES  OF  A&ROGRAPHY 


FIG.   103. 


CHANGE  IN  WINDS  NEAR  THE 
STORM  CENTER 


conditions  in  the 
vicinity  of  Gal- 
veston." 

In  the  1915 
storm  the  loss  of 
life  was  about  275, 
"whereas  in  1900 
the  loss  at  Gal- 
veston  and  vicin- 
ity alone  was  at 
least  6,000.  The 
great  difference  in 
favor  of  the  storm 
of  1915  was  due  in 
greatest  measure 
to  the  sea  wall 
which  was  con- 
structed by  the 
city  of  Galveston 
shortly  after  the 
flood  of  1900.  There  can  be  no  question  but  that  this  wall 
saved  the  lives  of  thousands  of  people.  It  should  also  be 
remarked  that  the  personal  efforts  of  the  official  in  charge  of 
the  local  office  of  the  Weather  Bureau  at  Galveston  were 
instrumental  in  saving  the  lives  of  hundreds  of  dwellers  on 
Galveston  Island.  The  official  at  Galveston  sent  out  men  on 
motorcycles  to  all  places  that  could  be  reached  on  Galveston 
Island,  who  warned  the  inhabitants  of  the  coming  of  the  storm 
and  impressed  upon  them  the  fact  that  unless  they  imme- 
diately sought  places  of  safety  they  would  surely  lose  their 
lives.  Subsequent  occurrences  confirmed  the  timeliness  and 
correctness  of  this  warning.  Much  commendation  is  also  due 
to  Prof.  W.  B.  Stearns,  cooperative  observer  and  storm  warning 
displayman  at  Seabrook,  Tex.  Upon  the  receipt  of  the  first 
general  warning  on  Sunday,  August  15,  and  again  Monday, 
August  16,  Professor  Stearns  personally  visited  all  the  in- 
habitants at  the  low  places  in  Seabrook  and  warned  them  to 
remove  to  places  of  safety  on  higher  ground.  There  were 
formerly  88  houses  on  the  bay  front,  and  now  there  are  3. 


FLOODS  AND  NOTABLE  STORMS 


257 


Nevertheless,  all  of  the  former  occupants  were  saved,  except 
two,  who  apparently  did  not  heed  the  warnings.  Outside  of 
Galveston  the  greatest  loss  in  life  probably  occurred  in  the 
vicinity  of  Texas  City,  across  the  bay  from  Galveston." 

68.  The  New  Orleans  storm  of  September  29,  1915.  It  is 
seldom  that  three  hurricanes  approach  the  Gulf  Coast  within  a 
period  of  six  weeks,  yet  such  was  the  case  in  August  to  Octo- 
ber in  1915.  The  Galveston  storm  of  August  described  above 
was  followed  by  a  storm  of  moderate  severity  which  passed 


FIG.  104.     RAINFALL  DISTRIBUTION  IN  WEST  INDIAN  HURRICANE, 
SEPTEMBER  28,   29,   30,   1915 


Cline 


inland  near  the  mouth  of  the  Apalachicola  River  on  Septem- 
ber 4.  This  was  followed  by  a  storm  of  marked  intensity 
which  passed  near  New  Orleans  on  September  29,  described  by 

18 


258  THE  PRINCIPLES  OF  ABROGRAPHY 

Cline  in  the  Monthly  Weather  Review  for  September,  1915,  as 
"the  most  intense  hurricane  of  which  we  have  record  in  the 
history  of  the  Mexican  Gulf  Coast."  An  exhaustive  and  most 
instructive  study  of  the  storm  has  been  made  by  Cline ;  and  a 
more  general  discussion  given  by  Bowie  in  a  special  Bulletin. 
The  storm  was  first  noted  by  the  forecast  officials  on  September 
22  in  the  doldrums,  in  latitude  15°  N.  and  longitude  64°  W.  By 
the  night  of  September  29  the  center  was  over  New  Orleans, 
Lowest  surpassing  in  intensity  the  Galveston  storm  of 

pressure  August.     The  lowest  pressure,  reduced  to  sea  level 

952  kbs.  an(j  correcteci  for  standard  gravity,  was  952  kilo- 

bars  (28.11  inches),  which  is  the  lowest  reading  ever  recorded 
at  a  Weather  Bureau  station.  The  extreme  wind  velocity 
was  approximately  58  meters  per  second  (130  miles  per  hour) 
from  the  east.  Fig.  102  shows  the  sea-level  pressure  at 
New  Orleans  during  the  passage  of  the  storm;  Fig.  103,  the 
wind  changes  along  the  hurricane  track;  and  Fig.  104,  rainfall 
distribution  in  the  various  parishes  of  Louisiana  near  the  path 
of  the  center. 


CHAPTER  XVII 

FROSTS 

69.  The  relation  between  the  surface  flow  of  air  and 
frosts.  Only  in  recent  years  have  aerographers  given  much 
attention  to  the  slow-moving  currents  of  the  lower  strata  of 
the  atmosphere.  These  strata  differ  greatly  from  the  whirls 
and  cataracts  of  both  high  and  low  levels  which  we  familiarly 
know  as  the  " winds."  The  larger  and  more  energetic  air 
streams  play  a  part  in  the  formation  of  frost,  chief  factor 
and  their  importance  in  this  regard  is  not  to  be  in  frost 
underestimated.  However,  it  is  a  slow  surface 
flow,  almost  a  creeping,  of  the  air  near  the  ground  which 
chiefly  controls  the  temperature  there  and  is  all-important  in 
frost  formation.  It  is,  therefore,  of  some  importance  to 
study  the  conditions  which  bring  about  this  slow  movement 
or  displacement  of  air.  It  is  true  that  there  are  times  when, 
owing  to  thorough  mixing  and  ventilation,  there  is  little 
opportunity  for  slow  displacement;  then  the  temperature  will 
fall  to  low  points  and  damage  from  frost  result.  But  such 
conditions  are  more  properly  described  as  cold  waves  (though 
the  term  is  somewhat  misleading),  or  " freezes."  In  such 
cases  there  is  an  unusual  loss  of  heat  by  direct  convection 
and  a  transfer  of  masses  of  cold  air.  Strictly  speaking, 
frosts  are  connected  with  temperature  inversions  brought 
about  by  a  vertical,  rather  than  a  horizontal,  movement  of 
the  air;  and  their  problem  is,  therefore,  essentially 
one  of  local  air  drainage.  The  expression  "local  drainages 
air  drainage"  requires  some  defining.  So  far  as 
known,  it  was  first  used  by  the  author1  in  explaining  frost. 
It  was  there  shown  that  in  the  valleys  of  California  a  well- 
marked  flow  of  the  surface  air  can  be  traced  and  utilized  in 
forecasting  frosts.  The  condensed  water  vapor  or  fog  can  be 
seen  drifting  into  the  valleys  or  settling  in  the  low  places. 
There  are  well-marked  stream  lines,  and  one  is  led  to  believe 

1(<  Frost  Fighting,"  Bulletin  No.  29,  U.  S.  Weather  Bureau,  1900. 

259 


260  THE  PRINCIPLES  OF  A$ROGRAPHY 

that  the  mixture  of  air  and  water  vapor  of  a  given  tempera- 
ture, say  275°A.,  cools  or  is  chilled  by  contact  with  the 
hilltops,  and  under  the  influence  of  gravity  and  other 
causes  flows  down  the  slopes.  The  term  "air  drainage" 
has  been  objected  to.  A  recent  writer  has  insisted  that  the 
flow  of  air  down  a  hillside  is  not  comparable  to  a  flow  of 
water,  since  water  is  an  incompressible  fluid  where  flow 
would  be  determined  by  gravity  alone,  while  air  is  a  com- 
pressible gas;  and  further  because  water  in  place 
may  fl°w  awaY>  leaving  the  space  it  occupied 
vacant.  But  this  writer  forgets  that,  as  von 
Bezold  and  others  have  shown,  any  substance  will  rise  or  fall 
without  additional  cooling  or  warming  ' '  when  it  forms  part  of 
an  endless  chain  that  glides  friction  less  over  a  roller  and  to 
which  there  has  once  been  given  a  velocity,  no  matter  how 
small." 

It  is  well  known  that  soon  after  sunset,  valleys  and  low 
places  serve  as  catchment  basins  for  slow-moving  air,  denser 
and  colder  than  that  above.  The  hilltops,  the  terraces,  and 
even  the  mountain  tops,  if  not  too  high,  are  in  contact  with 
air  of  higher  temperature,  which  must  be  either  an  indraft 
from  warm  surrounding  strata  or  the  displaced  air  from 
below.  How  the  circulation  begins  and  how  it  is  maintained 
are  not  clearly  understood;  and,  unfortunately,  we  have  no 

instruments  sufficiently  sensitive  to  record  this. 
Likelihood  of      _,  . .  ,-,1111,1  •  r 

frost  greater      The  cooling  of  the  lower  levels,  the  warming  ot 

where  air  is  the  Upper  levels,  and  the  existence  of  an  inver- 
sion are  evidently  not  the  result  of  a  single  cause. 
But  one  fact  stands  out  strongly  in  all  the  investigations 
thus  far  made;  and  that  is,  where  the  air  is  in  motion  there  is 
less  likelihood  of  frost  than  where  the  air  is  stagnant. 

70.  The  various  processes  in  frost  formation.  One 
cause  of  circulation  is  the  fact  that  the  slopes,  especially 
those  facing  west  of  southwest,  have  been  heated  by  insolation 
during  the  day,  and  therefore  radiate  more 
rapidly.  Radiation  is  a  function  of  the  absolute 
temperature,  and,  other  things  being  equal,  the 
surface  that  is  warmest  will  radiate  at  a  more  rapid  rate. 
The  energy  thus  radiated  is  not  absorbed  by  any  layer  of 


FROSTS 


261 


vapor  or  by  dust 
particles,  as  may 
be,  and  generally 
is,  the  case  in  the 
lower  levels.  The 
valleys  and  low 
levels  also  lose 
heat  by  radiation; 
but  soon  after  sun- 
set there  is  formed 
a  thin  blanket  of 
condensed  vapor, 
which  interferes 
with  free  radiation 
and  checks  the  rate 
of  cooling.  The 
air  at  the  higher 
level  is  drained  of 
its  load  of  vapor 
and  dust  nuclei, 
becoming  more  and 
more  like  a  pure 
gas  and  permitting 
freer  radiation.  A 
mixture  of  air  and 
vapor  per  unit 
volume  is  lighter 
than  dry  air;  and 
thus  moist  air 
naturally  tends  to 
rise.  The  con- 
densed vapor,  how- 
ever,  must  be 
regarded  in  a  dif- 
ferent light  from 
the  vapor  before 
condensation.  In 
condensing,  and 
also,  but  to  a  less 


262  THE  PRINCIPLES  OF  AEROGRAPHY 

degree,  in  congealing  into  frost  flakes,  heat  is  set  free  in  the 
sense  that  molecular  energy  is  decreased.  This  heat  is  not 
Formation  and  snown  as  a  direct  rise  in  temperature,  but  does 
effect  of  con-  serve  to  prevent  any  fall  in  temperature,  such  as 
vapor  expansion  due  to  rising  would  produce.  Thus 
we  have  near  the  ground  an  increasing  load  of  condensed 
vapor,  or  vapor  near  the  condensing  point,  which  either 
crystallizes  as  frost  with  further  cooling  or  is  carried  away 
by  convective  currents. 

We  see,  then,  that  there  are  various  conflicting  processes: 
we  have  gain  and  loss  of  heat  by  radiation,  the  upper  slopes 
losing  heat  by  radiation  and  the  lower  air  masses  gaining 

heat;  retardation  or  acceleration  of  rate  of  tem- 
pro"essesg  perature  change  with  the  change  in  state  of  the 

water  vapor;  direct  gain  or  loss  of  heat  by  con- 
vection, or  the  actual  translation  of  cold  and  warm  air  masses; 
and,  finally,  some  slight  gain  by  conductivity. 

Unfortunately,  the  word  ''frost"  has  been  used  as  the 
equivalent  for  lowest  temperature,  whereas  it  is  more  properly 
Water  vapor  simply  an  indication  of  the  existence  of  sufficient 
in  frost  water  vapor,  some  of  which  has  been  changed 

into  ice  in  the  form  of  spicular  crystals.  Such 
deposit  does  not  necessarily  indicate  the  place  of  lowest 
temperature;  for  with  other  than  saturation  conditions,  lower 
temperatures  may  prevail  without  the  formation  of  crystals. 
Some  good  .illustrations  of  the  inversion  of  temperature  are 
shown  in  the  accompanying  diagrams  (Figs.  105,  106,  and 
107). 

Fig.  105  illustrates  a  remarkable  inversion  which  occurred 
January  5,  1904,  when  the  temperature  at  the  valley  station 

at  Blue  Hill  fell  to  233°A.  These  records  show 
hwTrPstonsUre  ^ow  great  the  difference  in  temperature  may  be, 

at  different  levels  on  the  side  of  a  hill,  during  the 
early  morning  hours  on  a  still  winter  day.  This  is  the 
greatest  inversion  recorded  at  Blue  Hill.  There  was  also  an 
inversion  of  similar  character  on  the  succeeding  night.  In 
Fig.  106  is  shown  an  inversion  occurring  on  February  25,  1914, 
which  is  of  special  interest,  since  the  effective  cause  of  cooling 
did  not  begin  soon  after  sunset,  as  is  the  case  with  most 


FROSTS 


263 


inversions.  In 
fact,  it  did  not 
manifest  itself 
until  long  after 
midnight.  The 
records  illustrate 
how  the  air  is 
stratified  at  times 
of  frost,  the  cold- 
est, densest  air 
settling  in  the 
valley,  and  being 
displaced  during 
the  forenoon. 
Fig.  107  illus- 
trates typical 
early  fall  and  late 
spring  inversions, 
of  special  interest 
to  gardeners  and 
truck  farmers. 
In  the  records  is 
shown  the  drain- 
age of  cold  air  to 
the  lower  levels 
and  the  likelihood 
of  injurious  frost 
while  the  higher 
levels  are  im- 
mune. This  ex- 
plains why  frosts 
are  so  frequent  in 
the  lowlands  as 
compared  with 
the  slopes.  Figs. 
108  and  109  show 
frost  on  glass. 

In  all  these  it 
will     be     noticed 


264  THE  PRINCIPLES  OF  A&ROGRAPHY 

that   there    is    a    rapid    rise   in    temperature    at    the   lowest 
level,  shortly  after  sunrise,  and  a  slow  rise  at  the  base  and 
.  a  still  slower  rise  at  the  summit.     The  respect- 

temperature      ive  heights  of  the  three  stations  are:  valley,   18 

feVeT  10W6St  meters'  base  of  hil1'  66  meters;  and  summit,  200 
meters  above  sea  level.  This  rapid  rise  at  the 
valley  level  is  significant,  for  it  indicates  that  with  the  for- 
mation of  convective  currents  due  to  insolation  there  is  air 
movement  and  effective  heating.  One  might  infer  from 
this  that  the  cause  of  cooling  is  not  radiation,  but  the  rapid 
dying  out  of  convectional  currents  near  the  ground  after 
sunset,  any  uprise  of  air  being  counterbalanced  by  the 
slow  down-moving  air  from  the  higher  slopes;  not  from 
the  mass  of  air  itself,  which  in  the  main  is  warmer,  as  we 
have  seen,  and  which  remains  relatively  warm  throughout 
the  night,  since  radiation  from  air  is  less  rapid  than  from  the 
hillsides.  Apparently,  then,  there  is  a  slow  flow  of  air  down 
Hi  h  tem  sides  into  the  valley.  It  seems  plain  from 

perature  at        these  inversions   that   the  principal   reason   why 

high  levels  ^he  summit  temperature  remains  high  during  the 
due  to  mixing  .  ,,  •  ,  £  ,,  £  -, 

night  is  because  of  the  existence  of  a  moderate 

air  movement  and  consequent  mixing.  Frequent  eye  observa- 
tion of  the  rate  of  ascent  of  smoke  from  a  valley  on  quiet, 
clear  afternoons  may  enable  one  to  surmise  that  inversion 
already  exists  to  some  degree  and  thus  accurately  foretell 
night  inversion  and  the  frosts  of  the  next  morning.  Any 
change  in  velocity  or  direction  of  air  flow  is  accompanied 
with  fluctuation  in  temperature. 

There  are  easily  recognizable  certain  types  of  storm  move- 
ment which  are  followed  by  frosts.     Gusty  northwest  winds, 

dying  out  at  sunset,  with  unclouded  skies  and 
storm  °f  l°w  and  decreasing  humidity  above  100  meters 

movement  but  increasing  in  the  lower  levels,  are  significant 
frosts  mg  local  conditions  favoring  frost.  Some  writers 

have  made  use  in  their  frost  discussions  of  the 
adiabatic  rate  of  fall  of  temperature,  which  is  0.98  degree 
for  each  hundred  meters'  rise;  but  no  such  conditions  have 
been  found  to  occur  at  times  of  frost.  On  the  contrary,  as 
we  have  seen  above,  there  is  gain  and  loss  of  heat  in  various 


FROSTS 


265 


ways,  and  adia- 
batic  equilibrium 
is  out  of  the 
question.  Neither 
the  adiabatic  rate 
for  dry  air  nor 
that  for  saturated 
air  holds  from  the 
ground  up  to  200 
meters.  Instead 
of  a  fall,  there  is 
a  rise  in  temper- 
ature. 

As  the  tempera- 
ture  rises  the 
humidity  falls,  as 
a  general  rule. 
Unfortunately,  our 
instruments  for 
recording  are  un- 
satisfactory. 
Relative  humidity, 
standing  by  it- 
self and  as  ordi- 
narily expressed,  is 
very  misleading; 
in  fact,  it  means 
a  ratio  in  which 
one  term  is 
suppressed.  No 
proper  study  of 
frost  or  tempera- 
ture inversion  can 
be  made  without 
a  full  and  definite 
knowledge  of  the 
behavior  of  the 
water  vapor  and 
dust  content  of 


266 


THE  PRINCIPLES  OF  ARROGRAPHY 


deficit 
recorder" 


FIG.   108.     FROST  CRYSTALS 


the  atmosphere.  A  form  of  instrument  devised  by  the 
author  is  a  decided  improvement  over  the  usual  form  of 
"Saturation  hygrograph.  It  is  known  as  a  ''saturation  deficit 
recorder."  It  gives  a  continuous  record  of  the 
weight  of  the  vapor  in  grams  per  unit  volume. 
The  temperatures  are  given  in  degrees  absolute,  but  the 
record  sheet  also  is  graduated  to  show  the  saturation  weights 
for  each  degree.  The  thermograph  part  of  the  instrument 
(Fig.  110),  therefore,  records  the  appropriate  weight  of  the 
water  vapor  per  cubic  meter  at  saturation,  and  the  hygro- 
graph part  gives  the  existing  percentage.  The  difference 
between  the  two  is  the  saturation  deficit,  a  quantity  that 
may  be  used  to  advantage  in  discussions  of  frost  formation 
or  in  the  more  general  problem  of  the  change  in  a  given 
volume  of  moist  air  as  it  rises  or  falls  or  is  transported 


FROSTS 


267 


from  a  region  of  high  to  a  region  of  low  pressure.  It  is 
regrettable  that  we  have  no  method  of  recording  continu- 
ously what  von  Bezold  has  termed  the  ' '  mixing  ratio  ' '  or 
the  mass  of  vapor  mixed  with  a  unit  mass  of  dry  air 
expressed  as  a  fraction  of  this  latter  unit;  and  that  we 
have  no  way  of  ascertaining  what  the  same  investigator  calls 
"  specific  humidity,"  or  the  quantity  of  vapor  in  a  unit  mass 
of  moist  air  expressed  in  fractional  parts  of  this  unit.  More- 
over, there  is  a  difference  between  vapor  pressure  _. 
and  absolute  humidity,  although  the  two  are  sure  and 

often  considered  as  equivalent.     The  record  of       ?bso.1?.tf 

.  .  humidity 

the   mixing   ratio   would   be   important   in   frost 

work,  since,  for  any  given  change  of  level,  the  change  in  the 
mixing  ratio  would  give  the  quantity  of  ice  deposited,  —  not 


FIG.   109.     FROST  CRYSTALS 


McAdie 


268 


THE  PRINCIPLES  OF  A&ROGRAPHY 


FIG.  110.     SATURATION  DEFICIT  RECORDER 

This  instrument  records  the  temperature  in  degrees  absolute,  and  the  satura- 
tion weight  of  the  vapor  for  given  temperatures. 

in  this  case,  due  to  ascent  of  the  air,  but  to  contact  with  the 
hillsides  cooled  by  rapid  radiation  and  to  the  floor  of  the 
valley  which  also  acts  as  a  condensing  surface.  Moreover, 
the  quantity  of  water  (or  frost  crystals)  thus  separated  from 
the  air  would  give  an  indication  of  the  intensity  of  the  up-and- 
down  movement  of  the  air  and  its  content  of  vapor.  We 
have,  indeed,  in  frost  conditions  a  problem  somewhat  similar 
to  that  in  certain  cloud  formations, —  billows  and  bars,— 
less  pronounced,  but  none  the  less  phenomena  of  moving  air 
Close  prox-  strata  of  different  temperatures  and  densities  in 
imity  of  close  proximity.  It  may  be  pointed  out,  as 

unlike  strata  NeimOff  has  shown  in  his  reconstruction  of  the 
Hertz  diagram,  that  at  273°A.,  altitude  lines  run  parallel  to 
pressure  lines  at  equal  distances  from  each  other  for  equal 
pressure  changes;  hence  the  isothermal  change  of  altitude  at 
freezing  temperature  is  proportional  to  the  quantity  of  water 
present.  Practically  1  gram  of  freezing  water  corresponds 
to  a  change  of  level  of  27  meters. 

A  more  direcj  form  of  instrument  is  such  as  that  shown  in 
Fig.  111.  There  is  continuously  recorded  not  only  the  tem- 
perature but  also  the  temperature  of  evaporation.  The 
pressure  of  the  aqueous  vapor  can  be  readily  determined 
from  the  formula  given  by  Ferrel1  or  obtained  directly 
from  Smithsonian  Physical  Table  No.  150,  1914,  p.  157. 

1  Report  of  Chief  Signal  Officer,  1886,  App.  24. 


FROSTS 


269 


FIG.  111. 


THERMOGRAPH  FOR  DRY  AND  WET  BULB  READINGS 

Foxboro  type. 


The     dew    point,     relative    humidity,     and    humidity    term 
0.378,    which  occurs  in  the  formula  for  density  of  air  con- 
taining aqueous  vapor  at  pressure,  can  be  easily 
obtained.     This  instrument  gives  more  accurate     thermograph 
readings  if  the  two  thermometers  are  placed  near 
a   small    fan   or  other  ventilating   device.      It   is   preferable 
that  this  instrument  be  not  inclosed  in  the  usual  louvered 
shelter.     It  should  not  be  placed  at  the  customary  elevation 
of  two  meters  above  the  ground,  but  as  near  the  ground  as 
possible. 

It  is  also  necessary  to  pay  special  attention  to  the  purity 
of  the  water  and  the  cleanness  of  the  muslin  used  on  the 


270  THE  PRINCIPLES  OF  A&ROGRAPHY 

wet  bulb  for  evaporating  the  film  of  water.  The  pressure 
of  saturated  aqueous  vapor  varies  somewhat  at  temperatures 
near  273°A.,  the  variation  depending  upon  whether  the 
radiation  is  from  a  water  or  an  ice  surface.  The  following 
short  table  illustrates  this  difference: 


OVER    WATER 

OVER    ICE 

Temperature 
°A. 

Vapor  pressure 
mm. 

Temperature 
°A. 

Vapor  pressure 
mm. 

270 
271 
272 
273 

3.67 
3.95 
4.25 

4.58 

270 
271 
272 
273 

3.56 

3.88 
4.21 

4.58 

We  have  seen  that  it  is  of  importance  to  obtain  reliable 
records  of  the  amount  of  water  vapor  present,  and,  if  pos- 
sible, the  changes  which  this  quantity  undergoes.  One 
source  of  cooling  is  the  abstraction  of  a  considerable  quan- 
tity of  moisture  from  the  air.  A  copious  deposit  of  frost, 
however,  does  not  necessarily  indicate  the  region  of  lowest 
temperature. 

71.  Conversion  table  for  frost  work.  Orchard  heaters, 
evaporators,  and  frost  protectors  of  various  forms  have 
come  into  such  widespread  use  that  a  convenient  table  for 
the  quick  conversion  of  heat  units  into  power  units,  and 
vice  versa,  seems  to  be  much  needed. 

It  may  be  pointed  out  that  the  British  thermal  unit  is 

the  quantity  of  heat  required  to  raise  the   temperature  of 

1    pound   of   pure   water   at   maximum   density, 

fhermal  unit  39-10  R>  10  F-  This  is  the  unit  frequently  used 
by  engineers  in  this  country  and  Great  Britain, 
but  it  is  desirable  that  the  old  English  units  and  the  Fahrenheit 
scale  be  used  as  little  as  possible.  A  British  thermal  unit  is 
equal  to  0.252  calorie  and  also  equal  to  777.5  foot-pounds. 
One  therm  will  raise  the  temperature  of  1  gram  of  water 
1°C.;  1,000  therms  equal  1  calorie,  equal  to  3.968  British 
thermal  units. 

In  problems  connected  with  the  heat  of  water,  it  should 
be  remembered  that  the  total  heat  is  the  latent  heat  plus  the 
sensible  heat.  The  total  heat  required  to  evaporate  water 


FROSTS  271 

at  a  given  temperature  is  1,059.7  +  0.428  T,  where  T  is  a 
given  temperature.  This  holds  for  temperatures  between 
273°  A.  and  373°  A. 

In  changing  to  steam  at  373°A.  a  pound  of  water  at  373°  A. 
absorbs  970.4  British  thermal  units  and  the  total  heat  is 
therefore  1,150.4  British  thermal  units.  This  is  starting 
from  a  temperature  of  273°  A.  A  pound  of  ice  at  273°  A. 
requires  142.4  British  thermal  units  to  change  into  water  at 
273°  A. 

The  latent  heat  of  aqueous  vapor  may  be  found  from  the 
following  formula : 

£4  =  1,091.7-0. 572  fc 
where 

Ld  =  latent  heat; 
id  =  temperature  of  water. 

For  convenience  in  frost  work  the  following  may  be  used: 

1  kilowatt  hour  =  3,412.66  B.T.U. 
1  H.P.  =  746.3  watts. 
1H.P.  hour  =  2,544.6  B.T.U. 
1  B.T.U.  =  777.5  foot-pounds. 
1  B.T.U.  =0.252  calories. 
1  calorie  =  1,000  therms. 
1  calorie  =  3.968  B.T.U. 

1  calorie  per  kilogram  =  1.8  B.T.U.  per  pound. 
1  pound  of  air  at  32°  F.  occupies  about  12.4  cubic  feet. 
1  pound  of  water  at  212°F.  occupies  0.0161  cubic  feet. 
1  pound  of  steam  at  212°  F.  occupies  26.14  cubic  feet. 
1  pound  of  water  at  212°  F.  contains  181.8  B.T.U. 
1  pound  of  steam  at  212°  F.  contains  1,150.4  B.T.U. 
1  pound  of  ice  requires  143.8  B.T.U.  to  change  to  water. 
1  cubic  foot  of  water  at  212°  F.  weighs  59.84  pounds. 
1  cubic  foot  of  water  at  62°  F.  weighs  62.2786  pounds. 
1  cubic  foot  of  steam  at  212°  F.  weighs  0.03826  pound. 
1  cubic  foot  of  dry  air  at  32°  F.  weighs  568  grains. 
1  cubic  meter  of  dry  air  at  0°  C.  weighs  1,293.05  grams. 
Specific  heat  of  water,  1. 
Specific  heat  of  ice,  0.489. 

Specific  heat  of  water  vapor,  0.453  at  atmospheric  tempera- 
tures. 
Specific  heat  of  air,  0.241. 

Values  given  above  are  laboratory  values,  obtained  by 
using  distilled  water.  Ordinary  drinking  water  is  heavier 


272  THE  PRINCIPLES  OF  A&ROGRAPHY 

than  distilled  water,  because  of  matter  in  solution.  Salt 
water  is  also  heavier.  It  may  be  remarked  that  the  tempera- 
ture of  the  freezing  point  in  ordinary  use,  that  is,  273°  A.,  may 
not  hold  for  the  freezing  of  water  in  plant  life.  W.  N.  Shaw 
instances  one  plant  where  the  freezing  point  is  apparently 
268° A.  In  other  words,  the  change  of  water  from  the  liquid 
to  the  solid  state  under  natural  conditions  is  somewhat  differ- 
ent from  the  change  as  studied  in  a  laboratory. 


CHAPTER  XVIII 

SOLAR  INFLUENCES 

72.  The  source  of  radiant  energy.  The  earth  is,  rela- 
tively speaking,  an  insignificant  unit  in  the  solar  system. 
Fig.  112  illustrates  the  relative  sizes  of  sun  and  earth.  Further- 
more, the  solar  system  itself  is  an  insignificant  unit  in  the 
stellar  universe.  Astronomers  tell  us  that  the  solar  system 
is  moving  rapidly  toward  the  constellation  Hercules ;  but  no 
appreciable  effect  upon  the  earth's  atmosphere  is  known  to 
result,  nor  is  the  amount  of  energy  received  from  the  stars 
sufficient  to  produce  any  observable  effect.  Efforts  have 
been  and  are  being  made  to  measure  the  scattered  radiation 
of  the  sky;  but  as  yet  there  are  no  positive  results.  With 
the  radiant  energy  of  the  sun,  however,  it  is  different;  and 
here  we  have  to  deal  with  a  prime  mover.  The  mean  dis- 
Relative  posi-  tance  of  the  sun  from  the  earth  is  149,500,000 
tion  of  earth  kilometers  or  92,900,000  miles.  The  solar  paral- 
lax is  8.796  seconds  and  the  sun's  diameter 
1,392,000  kilometers,  or  865,000  miles.  The  velocity  of 
light  is  299,870  kilometers  per  second  (186,300  miles),  and  the 
time  required  to  traverse  the  mean  radius  of  the  earth's  orbit 
is  498.8  seconds.  The  visible  spectrum  comprises  light 
waves  ranging  in  wave  length  from  0.7/z  (0.0007  mm.),  the 
red  end,  to  0.4  ju  (0.0004  mm.),  the  violet  end.  At  only  a 
few  aerological  observatories  are  records  of  the  intensity  of 
solar  radiation  maintained.  Perhaps  one  of  the  most  serv- 
iceable records  is  that  made  at  Davos,  in  the  Swiss  Alps, 
where  continuous  records  have  been  obtained  by  C.  Dorno. 
At  this  mountain  station  a  continuous  photographic  record  of 
the  length  of  the  ultra-violet  spectrum  (that  is,  the  value  of 
the  shortest  waves  which  penetrate  the  atmosphere)  shows 
that  the  winter  sun  has  great  heating  effect,  but  apparently 
does  not  attain  a  maximum  in  the  other  end  of  the  spectrum. 
The  spring  sun  has  the  greatest  heat,  with  somewhat  greater 
amount  of  ultra-violet  radiation;  the  summer  sun,  much 

19  273 


274  THE  PRINCIPLES  OF  AEROGRAPHY 


From  Smithsonian  Report,  1913,  Hale 

FIG.   112.     DIRECT  PHOTOGRAPH  OF  THE  SUN 

A  dot  one  millimeter  in  diameter  would  represent  the  size  of  the  earth. 

heat  and  strongest  utra- violet;  and  the  autumn  sun,  much 
heat  and  much  ultra-violet.  Gockel  thinks  that  herein  lies 
Heat  and  ^he  explanation  of  the  "glacier  burn,"  that  is, 
ultra-violet  an  intense  ultra-violet  radiation.  One  point  of 
interest  is  that  the  ultra-violet  radiation  under- 
goes more  variation  than  the  heat ;  and  varies  greatly  with  the 
season,  so  that  a  single  day  in  summer  may  equal  a  winter 
month's  total. 

The  intensity  of  solar  radiation  is  measured  by  the  heat 


SOLAR  INFLUENCES 


275 


produced  when  a  given  surface  exposed  at  right  angles  to 
the  beam  entirely  absorbs  the  radiant  energy.  The  mean 
value  of  the  so-called  "solar  constant  of  radia-  "Solar 
tion"  has  been  fixed  by  Abbot  at  1.932  calories  constant  of 
per  square  centimeter  per  minute.  This  value  radiation" 
differs  materially  from  former  values,  especially  the  generally 
accepted  value  of  3  calories  as  given  in  many  textbooks.  If 
the  solar  constant  were  indeed  constant,  the  earth  would  receive 
in  a  year  something  like  one  million  million  million  million 
calories.  In  popular  terms  this  would  be  sufficient  heat  to  melt 
a  layer  of  ice  33  meters  thick  over  the  entire  surface  of  the  earth 
annually,  or  to  evaporate  1.66  X  1013  kilograms  of  water,  pro- 
vided there  were  no  atmosphere,  no  absorption,  and  no  reflection. 
73.  Variation  in  sunshine.  At  any  given  point  there 
must,  of  course,  be  variation  as  the  sun  changes  longitude. 
Thus  on  January  1,  when  the  longitude  is  1°,  the 
ratio  is  1.03;  on  March  1,  longitude  59°,  1.02;  on  variation  in 
July  1,  longitude  179°,  0.96;  on  September  1, 
longitude  240°,  0.98;  and  on  December  1,  longi- 
tude 330°,  1.03.  Thus  in  winter  the  value  is  larger  than  in 
summer.  The  duration  of  sunshine  can  be  determined  for 
any  given  latitude  from  the  hour  angle  converted  Variation 
into  mean  solar  time  and  then  multiplied  by  2.  with 
Considering  northerly  declination  positive,  and  longitude 
southerly  declination  negative,  we  have  for  example  in  lati- 
tude 42°  N.  the  following  values: 

DURATION  OF  SUNSHINE 


DECLINATION  OF  THE  SUN 

LENGTH  OF  DAY 

Hours              Minutes 

-23°  27' 

9               7 

-20° 

9              37 

-15° 

10              18 

-10° 

10 

56 

-  5° 

11 

33 

0° 

12 

9 

5° 

12 

45 

10° 

13 

22 

15° 

14 

1 

20° 

14 

43 

23°  27' 

15              14 

276 


THE  PRINCIPLES  OF  A&ROGRAPHY 


The  greatest  possible  duration  for  other  latitudes  is: 


Latitude 

0 

20° 

40° 

60° 

66° 

90° 

Maximum  insolation  .  . 

i2hr 

13h20' 

ISM' 

18h  52' 

24  h 

Q  months 

If  the  unit  of  insolation  be  the  amount  received  in  a  day 
at  the  equator  on  March  21,  then  for  given  latitudes  values 
will  vary  in  the  following  ratios: 


Latitude  
March  21  ... 

0 
1  0 

20° 
0  93 

40° 
0  76 

60° 
50 

North  Pole 
0 

South  Pole 
0 

June  21  

0.98 

1.04 

1.10 

1.09 

1.20 

n 

Sept.  23  

0.88 

0.94 

0  70 

0  30 

0 

0 

Dec.  21  . 

0.94 

0.68 

0.35 

0. 

0. 

1.28 

The  orbit  which  the  sun  appears  to  make  around  the  earth, 

but  which  in  reality  is  made  by  the  earth  around  the  sun,  is 

.  not  a  circle  but  an  ellipse  inclined  to  the  plane 

distance  of  the  equator.     The  speed  of  the  earth  is  not 

amTsun  earth   constant  5  an<^  instead  of  traveling  equal  distances 

in  equal  times,  the  distance  traveled  is  such  as 
to  make  the  areas  swept  over  by  the  line  joining  earth  and 
sun  equal  in  equal  times.  So  when  the  sun  is  nearest,  the 
earth  travels  fastest.  As  we  have  said,  the  sun  appears  to 
travel  in  a  plane  which  makes  an  angle  of  23°  with  the  plane 
of  the  equator. 

There  may  be  other  causes  of  variation  in  the  intensity  of 
solar  radiation — changes  which  may  be  of  solar  origin  and 
not  periodic.  Thus  the  monthly  mean  values  of  the  solar  con- 
stant from  1905  to  1912  have  been  compared  with  the  so-called 
"Wolff  sunspot  numbers"  for  the  same  months,  and  it  seems 
likely  that  increased  values  of  the  solar  constant  attend 

increased  sunspot  numbers.  In  the  report  of 
numbers  the  Astrophysical  Observatory  for  1913  it  is 

stated  that  there  is  an  increase  of  radiation,  at 
the  earth's  mean  distance  from  the  sun,  of  0.07  calorie  per 
square  centimeter  per  minute  with  an  increased  spottedness 
of  the  sun,  represented  by  100  Wolff  sunspot  numbers. 

Simultaneous  observations  at  Mount  Wilson  and  Bassour, 
Algeria,  indicate  that  fluctuations  in  solar-constant  values 


SOLAR  INFLUENCES  277 

found  in  California  in  earlier  years  may  now  be  explained 
not  as  local  phenomena  but  as  due  to  causes  outside  of 
the  earth ;  and  thus  we  may  conclude  that  the  sun 
is  a  variable  star,  having  not  only  a  periodicity  variable  star 
connected  with  the  periodicity  of  sunspots,  but 
also  an  irregular,  non-periodic  variation,  sometimes  running 
its  course  in  a  week  or  ten  days,  again  in  longer  periods,  and 
ranging  over  irregular  fluctuations  of  from  2  to  10  per  cent 
of  the  total.  It  has  also  been  shown  by  Abbot,  Fowle,  Kim- 
ball,  and  others  that  great  volcanic  eruptions  materially 
decrease  the  apparent  solar  radiation,  or  rather  that  atmos- 
pheric transmissibility  undergoes  marked  changes  with  conse- 
quent diminution  of  temperature.  Marked  changes  occurred 
in  1884-1886  (probably  connected  with  Krakatau)  and  again 
in  1903-1904. 

74.  Measurement  of  solar  radiation.  By  using  a  Callen- 
dar  pyrheliometer  and  an  eclipsing  screen,  the  total  radiation 
can  be  obtained  in  two  components,  one  representing  direct 
solar  radiation  and  the  other  the  diffuse  sky  radiation.  The 
total  radiation  per  square  centimeter  of  horizontal  surface 
with  the  clearest  sky  varies,  according  to  Kimball,  for  the 
particular  point  of  observation  (near  Washington),  from 
250  calories  a  day  (December  20)  to  765  calories  (June  10). 
In  general  the  radiation  received  on  clear  days  during  the 
first  half  of  the  year  exceeds  that  of  the  second  half  by 
8  per  cent,  probably  due  to  the  increased  water- vapor  content 
of  the  atmosphere  during  the  latter  period. 

The  total  radiation  received  with  the  clearest  sky  in  mid- 
day per  square  centimeter  of  horizontal  surface  varies  from  45 
calories  in  December  to  90  in  June.  When  clouds  are  near 
the  sun  but  do  not  obscure  it,  the  momentary  maximum 
rates  are  increased  by  about  0.15  calorie. 

The  diffuse  sky  radiation  received  on  a  horizontal  surface 
at  noon  averages  about  25  per  cent  of  that  from  the  sun. 

Expressed  in  units  of  work, 

1  calorie  per  minute  per  cm.2  represents  697  watts  per  m2.  ' 
90  calories  per  hour  (1^  per  minute)  represent  1  kilowatt  per  m2. 

The  radiation  received  on  a  square  meter  of  horizontal  surface 


278  THE  PRINCIPLES  OF  A&ROGRAPHY 

on  a  clear  day  in  midsummer  is,   therefore,  equivalent  to  5 
kilowatt  hours. 

Some  recent  measurements  are: 

American  University,  Washington,  D.C.,  on  December  24, 

1914,  with  the  sun  at  zenith  distance  62.5°,  an  intensity  of  1.48 
calories  per  minute  per  square  centimeter ;  and  on  February  28, 

1915,  with  the  sun  at  zenith  distance  57.5°  the  intensity  was 
1.50  calories.     At  Santa  Fe,  N.M.,  elevation  2,133  meters, 
and  in  an  arid  region,  a  maximum  of  1.64  calories  was  recorded 
with  a  zenith  distance  of  55°.     In  brief,  at  sea  level,  in  summer 
and  at  mid-day,  there  reaches  the  earth  each  second  .0225 
calorie  per  square  centimeter;   and  of  this   .0096  calorie  is 
scattered  or  absorbed  and  .006  calorie  re-radiated  from  the 
atmosphere.     The  amount  of  energy  varies  inversely  as  the 
square  of  the  distance  from  the  sun  also  with  the  angle  of 
incidence  of  the  rays,  and  according  to  duration. 

It  is  possible  that  there  is  in  the  upper  atmosphere  a  layer 
of  cosmical  dust  which  is  strongly  radioactive.  Simpson  has 
recently  pointed  out  that  the  measurements  of 
radiations  Vegard  and  Stormer  on  the  aurora  indicate  true 
radioactive  radiation  penetrating  the  atmosphere 
and  producing  the  same  results  as  if  the  atmosphere  were 
being  bombarded  from  the  outside  by  the  a  radiation,  which 
is  now  being  studied  in  so  many  physical  laboratories. 
Experiments  on  ionization  made  in  balloons  in  1914  show 
the  existence  of  a  strong  radiation.  This  may  help  explain 
the  nature  of  the  aurora.  The  average  height  of  the  bottom 
edge  of  the  aurora  as  determined  by  1920  measurements  in 
Norway  is  108  kilometers,  and  no  aurora  lower  than  85  kilo- 
meters was  noticed.  It  would  seem  that  the  cosmic  rays 
producing  the  aurora  are  in  two  groups  with  different  pene- 
trating power.  The  diffuse  arcs,  the  drapery,  and  more 
intense  displays  seem  to  be  of  the  same  physical  nature. 

A  pyranometer l  is  an  instrument  adapted  to  measuring  heat 

coming  from  or  going  to  space  above.     It  was 
Pyranometer       1      .     °    ,        A1  1  A,  .,   .  ,  .,    T^  r  , , 

devised  by  Abbot,  Aldrich,  and  Kramer  of  the 

Astrophysical  Observatory  of  the    Smithsonian    Institution, 

for  measuring  accurately  the  intensity  of  sky  light  by  day 

and  of  radiation  outward  toward  the  whole  sky  by  night. 

1For  complete  description  see  Smithsonian  Misc.  Coll.,  Vol.  66,  No.  7,  May,  1916. 


APPENDIX 


TABLE  1.     INCHES  INTO  MILLIMETERS 
1  inch  =  25.40005  mm. 


Inches 

.00 

.01 

.02 

.03 

.04 

.05 

.06 

.07 

.08 

.09 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

0.00 

0.00 

0.25 

0.51 

0.76 

1.02 

1.27 

1.52 

1.78 

2.03 

2.29 

0.10 

2.54 

2.79 

3.05 

3.30 

3.56 

3.81 

4.06 

4.32 

4.57 

4.83 

0.20 

5.08 

5.33 

5.59 

5.84 

6.10 

6.35 

6.60 

6.86 

7.11 

7.37 

0.30 

7.62 

7.87 

8.13 

8.38 

8.64 

8.89 

9.14 

9.40 

9.65 

9.91 

0.40 

10.16 

10.41 

10.67 

10.92 

11.18 

11.43 

11.68 

11.94 

12.19 

12.45 

0.50 

1270 

12.95 

13.21 

13.46 

13.72 

13.97 

14.22 

14.48 

14.73 

14.99 

0.60 

15.24 

15.49 

15.75 

16.00 

16.26 

16.51 

16.76 

17.02 

17.27 

17.53 

0.70 

17.78 

18.03 

18.29 

18.54 

18.80 

19.05 

19.30 

19.56 

19.81 

20.07 

0.80 

20.32 

20.57 

20.83 

21.08 

21.34 

21.59 

21.84 

22.10 

22.35 

22.61 

0.90 

22.86 

23.11 

23.37 

23.62 

23.88 

24.13 

24.38 

24.64 

24.89 

25.15 

1.00 

25.40 

TABLE  2.     FEET  INTO  METERS 
1  foot  =  0.3048006  meter 


Feet 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0 

m. 

0.000 

m. 

0.305 

m. 

0.610 

m. 

0.914 

m. 

1.219 

m. 

1.524 

m. 

1.829 

m. 

2.134 

m. 

2.438 

m. 

2.743 

10 

20 
30 
40 

3.048 
6.096 
9.144 
12.192 

3.353 
6.401 
9.449 
12.497 

3.658 
6.706 
9.754 
12.802 

3.962 
7.010 
10.058 
13.106 

4.267 
7.315 
10.363 
13.411 

4.572 
7.620 
10.668 
13.716 

4.877 
7.925 
10.973 
14.021 

5.182 
8.230 
11.278 
14.326 

5.486 
8.534 
11.582 
14.630 

5.791 
8.839 
11.887 
14.935 

50 

60 
70 

80 
90 

15.240 

18.288 
21.336 
24.384 
27.432 

15.545 
18.593 
21.641 
24.689 
27.737 

15.850 
18.898 
21.946 
24.994 
28.042 

16.154 
19.202 
22.250 

25.298 
28.346 

16.459 
19.507 
22.555 
25.603 
28.651 

16.764 
19.812 

22.860 
25.908 
28.956 

17.069 
20.117 
23.165 
26.213 
29.261 

17.374 
20.422 
23.470 
26.518 
29.566 

17.678  17.983 
20.72621.031 
23.77424.079 
26.82227.127 
29.87030.175 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

200 
300 
400 

30.48 
60.96 
91.44 
121.92 

33.53 
64.01 
94.49 
124.97 

36.58 
67.06 
97.54 
128.02 

39.62 
70.10 
100.58 
131.06 

42.67 
73.15 
103.63 
134.11 

45.72 
76.20 
106.68 
137.16 

48.77 
79.25 
109.73 
140.21 

51.82 
82.30 
112.78 
143.26 

54.86 
85.34 
115.82 
146.30 

57.91 

88.39 
118.87 
149.35 

500 

600 
700 
800 
900 

152.40 

182.88 
213.36 
243.84 
274.32 

155.45 
185.93 
216.41 
246.89 
277.37 

158.50 
188.98 
219.46 
249.94 
280.42 

161.54 
192.02 
222.50 

252.98 
283.46 

164.59 
195.07 
225.55 
256.03 
286.51 

167.64 

198.12 
228.60 
259.08 
289.56 

170.69 
201.17 
231.65 
262.13 
292.61 

173.74 
204.22 
234.70 
265.18 
295.66 

176.78 
207.26 
237.74 

268.22 
298.70 

179.83 
210.31 
240.79 
271.27 
301.75 

1000 

304.80 

279 


280 


APPENDIX 


TABLE  3.     MILES  INTO  KILOMETERS 
1  mile  =  1.609347  kilometers 


Miles 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

km. 

km. 

km. 

km. 

km. 

km. 

km. 

km. 

km. 

km. 

0 

0 

2 

3 

5 

6 

8 

10 

11 

13 

14 

10 

16 

18 

19 

21 

23 

24 

26 

27 

29 

31 

20 

32 

34 

35 

37 

39 

40 

42 

43 

45 

47 

30 

48 

50 

51 

53 

55 

56 

58 

60 

61 

63 

40 

64 

66 

68 

69 

71 

72 

74 

76 

77 

79 

50 

80 

82 

84 

85 

87 

89 

90 

92 

93 

95 

60 

97 

98 

100 

101 

103 

105 

106 

108 

109 

111 

70 

113 

114 

116 

117 

119 

121 

122 

124 

•  126 

127 

80 

129 

130 

132 

134 

135 

137 

138 

140 

142 

143 

90 

145 

146 

148 

150 

151 

153 

154 

156 

158 

159 

100 

161 

TABLE  4.     KILOMETERS  INTO  MILES 
1  kilometer  =  0.621370  mile 


Kilometers 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Miles 

Miles 

Miles 

Miles 

Miles 

Miles 

Miles 

Miles 

Miles 

Miles 

0 

0.0 

0.6 

1.2 

1.9 

2.5 

3.1 

3.7 

4.3 

5.0 

5.6 

10 

6.2 

.  6.8 

7.5 

8.1 

8.7 

9.3 

9.9 

10.6 

11.2 

11.8 

20 

12.4 

13.0 

13.7 

14.3 

14.9 

15.5 

16.2 

16.8 

17.4 

18.0 

30 

18.6 

19.3 

19.9 

20.5 

21.1 

,21.7 

22.4 

23.0 

23.6 

24  2 

40 

24.9 

25.5 

26.1 

26.7 

27.3 

28.0 

28.6 

29.2 

29.8 

30^4 

50 

31.1 

31.7 

32.3 

32.9 

33.6 

34.2 

34.8 

35.4 

36.0 

36.7 

60 

37.3 

37.9 

38.5 

39.1 

39.8 

40.4 

41.0 

41.6 

42.3 

42.9 

70 

43.5 

44.1 

44.7 

45.4 

46.0 

46.6 

47.2 

47.8 

48.5 

49.1 

80 

49.7 

50.3 

51.0 

51.6 

52.2 

52.8 

53.4 

54.1 

54.7 

55.3 

90 

55.9 

56.5 

57.2 

57.8 

58.4 

59.0 

59.7 

60.3 

60.9 

61.5 

100 

62.1 

APPENDIX 


281 


TABLE  5.     INTERCONVERSION  OF  NAUTICAL  AND  STATUTE  MILES 
1  nautical  mile*  =  6080.27  feet 


Nautical  Miles 

Statute  Miles 

Statute  Miles 

Nautical  Miles 

1 

1.1516 

1 

0.8684 

2 

2.3031 

2 

1.7368 

3 

3.4547 

3 

2.6052 

4 

4.6062 

4 

3.4736 

5 

5.7578 

5 

4.3420 

6 

6.9093 

6 

5.2104 

7 

8.0609 

7 

6.0788 

8 

9.2124 

8 

6.9472 

9 

10.3640 

9 

7.8155 

*  As  defined  by  the  United  States  Coast  Survey. 


TABLE  6.     CONVERSION  OF  VELOCITIES 

Miles  per  hour  into  meters  per  second,  feet  per  second, 
and  kilometers  per  hour 


Miles 
per 
hour 

Meters 
per 
second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

Miles 
per 
hour 

Meters 
per 
second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

Miles 
per 
hour 

Meters 
per 
second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

0.0 

0.0 

0.0 

0.0 

12.0 

5.4 

17.6 

19.3 

24.0 

10.7 

35.2 

38.6 

0.5 

0.2 

0.7 

0.8 

12.5 

5.6 

18.3 

20.1 

24.5 

11.0 

35.9 

39.4 

1.0 

0.4 

1.5 

1.6 

13.0 

5.8 

19.1 

20.9 

25.0 

11.2 

36.7 

40.2 

1.5 

0.7 

2.2 

2.4 

13.5 

6.0 

19.8 

21.7 

25.5 

11.4 

37.4 

41.0 

2.0 

0.9 

2.9 

3.2 

14.0 

6.3 

20.5 

22.5 

26.0 

11.6 

38.1 

41.8 

2.5 

11 

3.7 

4.0 

14.5 

6.5 

21.3 

23.3 

3.0 

1.3 

4.4 

4.8 

15.0 

6.7 

22.0 

24.1 

26.0 

11.6 

38.1 

41.8 

3.5 

1.6 

5.1 

5.6 

15.5 

6.9 

22.7 

24.9 

26.5 

11.8 

38.9 

42.6 

4.0 

1.8 

59 

6.4 

16.0 

7.2 

23.5 

25.7 

27.0 

12.1 

39.6 

43.5 

4.5 

2.0 

6.6 

7.2 

16.5 

7.4 

24.2 

26.6 

27.5 

12.3 

403 

44.3 

5.0 

2.2 

7.3 

8.0 

17.0 

7.6 

24.9 

27.4 

28.0 

12.5 

41.1 

45.1 

5.5 

2.5 

8.1 

8.9 

17.5 

7.8 

25.7 

28.2 

28.5 

12.7 

41.8 

45.9 

6.0 

2.7 

8.8 

9.7 

18.0 

8.0 

26.4 

29.0 

29.0 

13.0 

425 

46.7 

6.5 

2.9 

9.5 

10.5 

18.5 

8.3 

27.1 

29.8 

29.5 

13.2 

43.3 

47.5 

7.0 

3.1 

10.3 

11.3 

19.0 

8.5 

27.9 

30.6 

30.0 

13.4 

44.0 

48.3 

7.5 

3.4 

11.0 

12.1 

19.5 

8.7 

28.6 

31.4 

30.5 

13.6 

44.7 

49.1 

8.0 

3.6 

11.7 

12.9 

200 

8.9 

29.3 

32.2 

31.0 

13.9 

45.5 

49.9 

8.5 

3.8 

12.5 

13.7 

2C.5 

9.2 

30.1 

33.0 

31,5 

14.1 

46.2 

50.7 

9.0 

4.0 

13.2 

14.5 

21.0 

9.4 

30.8 

33.8 

32.0 

14.3 

46.9 

51.5 

9.5 

4.2 

13.9 

15.3 

21.5 

9.6 

31.5 

34.6 

32.5 

14.5 

47.7 

52.3 

10.0 

4.5 

14.7 

16.1 

22.0 

9.8 

32.3 

35.4 

33.0 

14.8 

48.4 

53.1 

10.5 

4.7 

15.4 

16.9 

22.5 

10.1 

33.0 

36.2 

33.5 

15.0 

49.1 

53.9 

11.0 

4.9 

16.1 

17.7 

23.0 

10.3 

33.7 

37.0 

34.0 

15.2 

49.9 

54.7 

11.5 

5.1 

16.9 

18.5 

23.5 

10.5 

34.5 

37.8 

34.5 

15.4 

50.6 

55.5 

282 


APPENDIX 


TABLE  6.     CONVERSION   OF   VELOCITIES — Continued 

Miles  per  hour  into  meters  per  second,  feet  per  second, 
and  kilometers  per  hour 


Miles 
per 
hour 

Meters 
per 
second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

Miles 
per 
hour 

Meters 
per 
Second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

Miles 
per 
hour 

Meters 
per 
second 

Feet 
per 
second 

Kilome- 
ters per 
hour 

35.0 

15.6 

51.3 

56.3 

60.0 

22.4 

73.3 

80.5 

64.0 

28.6 

93.9 

103.0 

35.5 

15.9 

52.1 

57.1 

50.5 

22.6 

74.1 

81.3 

64.5 

28.8 

94.6 

103.8 

36.0 

16.1 

52.8 

57.9 

51.0 

22.8 

74.8 

82.1 

65.0 

29.1 

95.3 

104.6 

36.5 

16.3 

53.5 

58.7 

51.5 

23.0 

75.5 

82.9 

65.5 

29.3 

96.1 

105.4 

37.0 

16.5 

54.3 

59.5 

52.0 

23.2 

76.3 

83.7 

66.0 

29.5 

96.8 

106.2 

37.5 

16.8 

55.0 

60.4 

66.5 

29.7 

97.5 

107.0 

38.0 

17.0 

55.7 

61.2 

52.0 

23.2 

76.3 

83.7 

67.0 

30.0      98.3 

107.8 

38.5 

17.2 

56.5 

62.0 

52.5 

23.5 

77.0 

84.5 

67.5 

30.2      99.0 

108.6 

39.0 

17.4 

57.2 

62.8 

53.0 

23.7 

77.7 

85.3 

68.0  *  30.4      99.7 

109.4 

39.5 

17.7 

57.9 

63.6 

53.5 

23.9 

78.5 

86.1 

68.5     30.6 

100.5 

110.2 

40.0 

17.9 

58.7 

64.4 

54.0 

24.1 

79.2 

86.9 

69.0  :  30.8    101.2 

111.0 

40.5 

18.1 

59.4 

65.2 

54.5 

24.4 

79.9 

87.7 

69.5  1  31.1     101.9 

111.8 

1 

41.0 

18.3 

60.1 

66.0 

55.0 

24.6 

80.7 

88.5 

70.0  f  31.3 

102.7 

112.7 

41.5 

18.6 

60.9 

66.8 

55.5 

24.8 

81.4 

89.3 

70.5  !  31.5 

103.4 

113.5 

42.0 

18.8 

61.6 

67.6 

56.0 

25.0 

82.1 

90.1 

71.0  !  31.7 

104.1 

114.3 

42.5 

19.0 

62.3 

68.4 

56.5 

25.3 

82.9 

90.9 

71.5     32.0 

104.9 

115.1 

43.0 

19.2 

63.1 

69.2 

57.0 

25.5 

83.6 

91.7 

72.0  j  32.2 

105.6 

115.9 

43.5 

19.4 

63.8 

70.0 

57.5 

25.7 

84.3     92.5 

72.5 

32.4 

106.3 

116.7 

i 

44.0 

19.7 

64.5 

70.8 

58.0 

25.9 

85.1 

93.3 

73.0 

32.6 

107.1 

117.5 

44.5 

19.9 

65.3 

71.6 

58.5 

26.2 

85.8 

94.1 

73.5  '  32.9 

107.8 

118.3 

45.0 

20.1 

66.0 

72.4 

59.0 

26.4 

86.5 

95.0 

74.0  ,  33.1 

108.5 

119.1 

45.5 

20.3 

66.7 

73.2 

59.5 

26.6 

87.3 

95.8 

74.5  !  33.3 

109.3 

119.9 

46.0 

20.6 

67.5 

74.0 

60.0 

26.8 

88.0 

96.6 

75.0 

33.5 

110.0 

120.7 

46.5 

20.8 

68.2 

74.8 

60.5 

27.0 

88.7 

97.4 

75.5 

33.8 

110.7 

121.5 

47.0 

21.0 

68.9 

75.6 

61.0 

27.3 

89.5 

98.2 

76.0 

34.0 

111.5 

122.3 

47.5 

21.2 

69.7 

76.4 

61.5 

27.5 

90.2 

99.0 

76.5 

34.2 

112.2 

123.1 

48.0 

21.5 

70.4 

77.2 

62.0 

27.7 

90.9 

99.8 

77.0 

34.4 

112.9 

123.9 

48.5 

21.7 

71.1 

78.1 

62.5 

27.9 

91.7 

100.6 

77.5 

34.6 

113.7 

124.7 

49.0 

21.9 

71.9 

78.9 

63.0 

28.2 

92.4 

101.4 

78.0 

34.9 

114.4 

125.5 

49.5 

22.1 

72.6 

79.7 

63.5 

28.4 

93.1 

102.2 

APPENDIX 


283 


TABLE  7.     PRESSURE 
Inches  of  Mercury  at  273°A.  and  45°  latitude,  to  Kilobars 

For  brevity,  the  fundamental  equations  may  be  written: 

£45  =980.624  cm/sec2. 

density  of  mercury  at  normal  freezing-point  of  water  =  13. 5959. 
1  mercury-inch  =33. 8660  kilobars;  1  millimeter  =  1.33320  kilobars. 
1000  kilobars  =  29.5306  mercury-inches  =  750.076  millimeters. 


Inches 
and 
Tenths 

.00 

.01 

.02 

.03 

.04 

.05 

.06 

.07 

.08 

.09 

KILOBARS 

27.0 

914.3 

914.6 

915.0 

915.3 

915.7 

916.0 

916.3 

916.7 

917.0 

917.4 

27.1 

917.7 

918.0 

918.4 

918.7 

919.0 

919.4 

919.7 

920.1 

920.4 

920.7 

27.2 

921.1 

921.4 

921.8 

922.1 

922.4 

922.8 

923.1 

923.4 

923.8 

924.1 

27.3 

924.5 

924.8 

925.1 

925.5 

925.8 

926.2 

926.5 

926.8 

927.2 

927.5 

27.4 

927.9 

928.2 

928.5 

928.9 

929.2 

929.5 

929.9 

930.2 

930.6 

930.9 

27.5 

931.2 

931.6 

931.9 

932.3 

932.6 

932,9 

933.3 

933.6 

933.9 

934.3 

27.6 

934.6 

935.0 

935.3 

935.6 

936.0 

936.3 

936.7 

937.0   937.3 

937.7 

27.7 

938.0 

938.3    938.7 

939.0 

939.4 

939.7 

940.0 

940.4    940.7 

941.1 

27.8 

941.4 

941.7 

942.1 

942.4 

942.8 

943.1 

943.4 

943.8J  944.1 

944.4 

27.9 

944.8 

945.1 

945.5 

945.8 

946.1 

946.5 

946.8 

947.2 

947.5 

947.8 

28.0 

948.2 

948.5 

948.8 

949.2 

949.5 

949.9 

950.2 

9505 

950.9 

951.2 

28.1 

951.6 

951.9 

952.2 

952.6 

952.9 

953.2 

953.6 

953.9 

954.3 

954.6 

28.2 

954.9 

955.3 

955.6 

956.0 

956.3 

956.6 

957.0 

957.3 

957.7 

958.0 

28.3 

958.3 

958.7 

959.0 

959.3 

959.7 

960.0 

960.4 

960.7 

961.0 

961.4 

28.4 

961.7 

962.1 

962.4 

962.7 

963.1 

963.4 

963.7 

964.1 

964.4 

964.8 

28.5 

965.1 

965.4 

965.8 

966.1 

966.5 

966.8 

967.1 

967.5 

967.8 

968.1 

28.6 

968.5 

968.8i  969.2 

969.5 

969.8 

970.2 

970.5 

970.9 

971.2 

971.5 

28.7 

971.9 

972.2    972.6 

972.9 

973.2 

973.6 

973.9 

974.2 

974.6 

974.9 

28.8 

975.3 

975.6    975.9 

976.3 

976.6 

977.0 

977.3 

977.6 

978.0 

978.3 

28.9 

978.6 

979.0 

979.3 

979.7 

980.0 

980.3 

980.7 

981.0 

981.4 

981.7 

29.0 

982.0 

982.4 

982.7 

983.0 

983.4 

983.7 

984.1 

984.4 

984.7 

985.1 

29.1 

985.4 

985.8 

986.1 

986.4 

986.8 

987.1 

987.5 

987.8 

988.1 

988.5 

29.2 

988.8 

989.1 

989.5 

989.8 

990.2 

990.5 

990.8 

991.2 

991.5 

991.9 

29.3 

992.2 

992.5 

992.9 

993.2 

993.5 

993.9 

994.2 

994.61  994.9 

995.2 

29.4 

995.6 

995.9 

996.3 

996.6 

996.9 

997.3 

997.6 

997.9 

998.3 

998.6 

29.5 

999.0 

999.3 

999.6 

1000.0 

1000.3 

1000.7 

1001.0 

1001.3 

1001.7 

1002.0 

29.6 

1002.4 

1002.7 

1003.0  1003.4 

1003.7 

1004.0 

1004.4 

1004.7 

1005.1 

1005.4 

29.7 

1005.7 

1006.1 

1006.4  1006.8 

1007.1 

1007.4 

1007.8 

1008.1 

1008.4 

1008.8 

29.8 

1009.1 

1009.5  1009.8  1010.1 

1010.5 

1010.8 

1011.2 

1011.5 

1011.8 

1012.2 

29.9 

1012.5 

1012.8 

1013.2 

1013.5 

1013.9 

1014.2 

1014.5 

1014.9 

1015.2 

1015.6 

30.0 

1015.9 

1016.2 

1016.6 

1016.9 

1017.3 

1017.6 

1017.9 

1018.3 

1018.6 

1018.9 

30.1 

1019.3 

1019.6  1020.0  1020.3 

1020.6 

1021.0 

1021.3 

1021.7 

1022.0 

1022.3 

30.2 

1022.7 

1023.0 

1023.3 

1023.7 

1024.0 

1024.4 

1024.7 

1025.0 

1025.4 

1025.7 

30.3 

1026.1 

1026.4 

1026.7 

1027.1 

1027.4 

1027.7 

1028.1 

1028.4 

1028.8 

1029.1 

30.4 

1029.4 

1029.81030.1 

1030.5 

1030.8 

1031.1 

1031.5 

1031.8 

1032.2 

1032.5 

284 


APPENDIX 


TABLE  7.     PRESSURE  —  Continued 
Inches  of  Mercury  at  273° A.  and  45°  latitude,  to  Kilobars 


Inches 

.00 

.01 

.02 

.03 

.04 

.05 

.06 

.07 

.08 

.09 

and 

Tenths 

KILOBARS 

30.5 

1032.8 

1033.2 

1033.5 

1033.8 

1034.2 

1034.5 

1034.9 

1035.2 

1035.5 

1035.9 

30.6 

1036.2 

1036.611036.9 

1037.2 

1037.6 

1037.9 

1038.2 

1038.6 

1038.9 

1039.3 

30.7 

1039.6 

1039.9 

1040.3 

1040.6 

1041.0 

1041.3 

1041.6 

1042.0 

1042.3 

1042.6 

30.8 

1043.0 

1043.3 

1043.7 

1044.0 

1044.3 

1044.7 

1045.0 

1045.4 

1045.7 

1046.0 

30.9 

1046.4 

1046.7 

1047.1 

1047.4 

1047.7 

1048.1 

1048.4 

1048.7 

1049.1 

1049.4 

Thousandths  of  an  Inch 


Inch 

.001 

.002 

.003 

.004 

.005 

.006 

.007 

.008 

.009 

Kilobars 

.0 

.1 

.1 

.1 

.2 

.2 

.2 

.3 

.3 

NOTE. — The  value  for  gravity  is  that  of  the  United  States  Coast  and  Geodetic 
Survey.  A  value  980.665  given  by  the  Bureau  of  Standards  was  adopted  in  1888 
by  the  International  Committee  on  Weights  and  Measures  and  has  since  been 
continued  for  convenience  although  it  is  a  conventional  standard  and  not  exactly 
equal  to  the  value  at  45°.  There  has  been  a  slight  change  in  the  value  for  the 
density  of  mercury.  The  differences  are  small.  See  Monthly  Weather  Review  for 
April,  1914,  p.  230,  article  by  R.  N.  Covert. 

TABLE  8.     MILLIMETERS  TO  KILOBARS 


Millimeters 

Kilobars 

Millimeters 

Kilobars 

1 
10 
100 
200 
,300 
400 

1.3 
13.3 
133.3 
266.6 
400.0 
533.3 

500 
600 
700 
800 
900 
1000 

666.6 
799.9 
933.2 
1066.6 
1200.0 
1333.3 

MILLIMETERS  TO  KILOBARS 

Base  1000  kbs.  or  one  million  dynes 


Millimeters 

0 

2 

4 

6 

8 

700 

-66.8 

-64.1 

-61.4 

-58.8 

-56.1 

710 

-53.4 

-50.8 

-48.1 

-45.4 

-42.8 

720 

-40.1 

-37.4 

-34.8 

-32.1 

-29.4 

730 

-26.8 

-24.1 

-21.4 

-18.8 

-161 

740 

-13.4 

-10.8 

-  8.1 

-  5.4 

-  2.8 

750 

-  0.1 

+  2.6 

+  5.2 

+  7.9 

+  10.6 

760 

+  13.2 

+  15.9 

+  .8.6 

+21.2 

+23.9 

770 

+26.6 

+29.2 

+31.9 

+  34.6 

+37.2 

780 

+39.9 

+42.6 

+45.2 

+47.9 

+50.6 

790 

+53.3 

+55.9 

+58.6 

+61.2 

+63.9 

APPENDIX 


285 


TABLE  9.     CONVERSION  OF  TEMPERATURES 


1 

Absolute 
(A.°) 

D 
V3 

£ 

.2?^ 

cU 

cu  -—  ' 

u 

Fahrenheit 

(F.°) 

i 
Reaumur 

(R.°) 

0> 
,§cP 

J< 

< 

<u 

1 

^r" 
•§y 

cT 

i 

Fahrenheit 
(F.°) 

Reaumur 
(R.°) 

373 

100 

212 

80 

303 

30 

86 

24 

68 

95 

203 

76 

2 

29 

84 

23 

63 

90 

194 

72 

1 

28 

82 

22 

58 

85 

185 

68 

300 

27 

81 

22 

53 

80 

176 

64 

48 

75 

167 

60 

300 

27 

81 

22 

43 

70 

158 

56 

299 

26 

79 

21 

38 

65 

149 

52 

98 

25 

77 

20 

33 

60 

140 

48 

97 

24 

75 

19 

28 

55 

131 

44 

96 

23 

73 

18 

325 

52 

126 

42 

295 

22 

72 

18 

24 

51 

124 

41 

94 

21 

70 

17 

23 

50 

122 

40 

93 

20 

68 

16 

22 

49 

120 

39 

92 

19 

66 

15 

21 

48 

118 

38 

91 

18 

64 

14 

320 

47 

117 

38 

290 

17 

63 

14 

19 

46 

115 

37 

89 

16 

61 

13 

18 

45 

113 

36 

88 

15 

59 

12 

17 

44 

111 

35 

87 

14 

57 

11 

16 

43 

109 

34 

86 

13 

55 

10 

315 

42 

108 

34 

285 

12 

54 

10 

14 

41 

106 

33 

84 

11 

52 

9 

13 

40 

104 

32 

83 

10 

50 

8 

12 

39 

102 

31 

82 

9 

48 

7 

11 

38 

100 

30 

81 

8 

46 

6 

310 

37 

99 

30 

280 

7 

45 

6 

9 

36 

97 

29 

79 

6 

43 

5 

8 

35 

95 

28 

78 

5 

41 

4 

7 

34 

93 

27 

77 

4 

39 

3 

6 

33 

91 

26 

76 

3 

37 

2 

305 

32 

90 

26 

275 

2 

36 

2 

4 

31 

88 

25 

74 

1 

34 

1 

286 


APPENDIX 


TABLE  10.     SUPPLEMENTARY  TO  TABLE  9 
CONVERSION  OF  MINUS  TEMPERATURES 


& 

3Z^ 

8<j 
,0^-" 
<j 

CD 
'O 

$ 

.SfK? 

cO 

^r 

Fahrenheit 
(F.°) 

Reaumur 
(R.°) 

3 

~°? 

in  <J 
<^ 

Centigrade 
(C.°) 

Fahrenheit 
(F.°) 

Reaumur 
(R.°) 

273 

0 

32 

0 

240 

-33 

-27 

-26 

72 

-1 

30 

-1 

39 

-34 

-29 

-27 

71 

-2 

28 

-2 

38 

-35 

-31 

-28 

37 

-36 

-33 

-29 

270 

-3 

27 

-2 

36 

-37 

-35 

-30 

69 

-4 

25 

-3 

68 

-5 

23 

-4 

235 

-38 

-36 

-30 

67 

-6 

21 

-5 

34 

-39 

-38 

-31 

66 

-7 

19 

-6 

33 

-40 

-40 

-32 

32 

-41 

-42 

-33 

265 

-8 

18 

-6 

31 

-42 

-44 

-34 

64 

-9 

16 

-7 

63 

-10 

14 

-8 

30 

-43 

-45 

-34 

62 

-11 

12 

-9 

29 

-44 

-47 

-35 

61 

-12 

10 

-10 

28 

-45 

-49 

-36 

27 

-46 

-51 

-37 

260 

-13 

9 

-10 

26 

-47 

-53 

-38 

59 

-14 

7 

-11 

58 

-15 

5 

-12 

225 

-48 

-54 

-39 

57 

-16 

3 

-13 

24 

-49 

-56 

-39 

56 

-17 

1 

-14 

23 

-50 

-58 

-40 

22 

-51 

-60 

-41 

255 

-18 

0 

-14 

21 

-52 

-62 

-42 

54 

-19 

-2 

-15 

53 

-20 

-4 

-16 

220 

-53 

-64 

-42 

52 

-21 

-6 

-17 

18 

-55 

-67 

-44 

51 

-22 

-8 

-18 

16 

-57 

-71 

-46 

14 

-59 

-74 

-48 

250 

-23 

-9 

-18 

12 

-61 

-78 

-49 

49 

-24 

-11 

-19 

48 

-25 

-13 

-20 

10 

-63 

-82 

-50 

47 

-26 

-15 

-21 

8 

-65 

-85 

-52 

46 

-27 

-17 

-22 

6 

-67 

-89 

-54 

4 

-69 

-92 

-55 

245 

-28 

-18 

-22 

2 

-71 

-96 

-57 

44 

-29 

-20 

-23 

43 

-30 

-22 

-24 

200 

-73 

-100 

-59 

42 

-31 

-24 

-25 

198 

-75 

-103 

-60 

41 

-32 

-26 

-26 

193 

-80 

-112 

-64 

APPENDIX 


287 


TABLE  10.     SUPPLEMENTARY  TO  TABLE  9 — Continued 
CONVERSION  OF  MINUS  TEMPERATURES 


-8 

'CD 

5_, 

•8 

•"S 

j_t 

o 

$ 

3 

0^ 

c^ 

"^cP 

(H 

£  ,-x 

13  o"^ 

M^ 

lr 

g<p 

*°<j 

|CJ 

J3fe 

§Q^ 

^< 

'•go 

3  6 

oj  '  —  ' 

dl  -^ 

<; 

o 

< 

o 

to 

^ 

188 

-85 

-121 

-68 

80 

-193 

-315 

-154 

183 

-90 

-130 

-72 

70 

-203 

-333 

-162 

60 

-213 

-351 

-170 

173 

-100 

-148 

-80 

50 

-223 

-369 

-178 

163 

-110 

-166 

-88 

40 

-233 

-387 

-185 

153 

-120 

-184 

-96 

143 

-130 

-202 

-104 

30 

-243 

-395 

-193 

133 

-140 

-220 

-112 

20 

-253 

-413 

-201 

10 

-263 

-431 

-209 

123 

-150 

-238 

-120 

5 

-268 

-440 

-213 

113 

-160 

-256 

-128 

4 

-269 

-452 

-215 

103 

-170 

-274 

-136 

3 

-270 

-454 

-216 

100 

-173 

-279 

-138 

2 

-271 

-456 

-217 

90 

-183 

-297 

-146 

1 

-272 

-458 

-218 

0 

-273.09 

-459.4 

-218.8 

0°A. 


2°A. 
4°A. 
10°A. 


20°A. 
79°A. 
86°A. 
79°A. 
91°A. 
90°A. 
163°A. 


CERTAIN  TEMPERATURES  ON  THE  ABSOLUTE  SCALE 

Professor  K.  Ormes  in  the  Cryogenic  Laboratory  at  Leiden 
found  that  at  the  temperature  of  nearly  absolute  zero, 
electrical  resistance  in  conductors  disappears.  At  2°  the 
electrical  resistance  of  a  thread  of  mercury  was  negligible. 

Onnes,  in  an  effort  to  obtain  solid  helium,  obtained  this 
temperature. 

Maximum  density  of  liquefied  helium. 

Boiling  point  of  helium. 

Effective  temperature  of  space.  At  an  elevation  of  80 
kilometers  (50  miles)  the  temperature  ranges  from  5°  to 
10°A. 

Boiling  point  of  hydrogen. 

Boiling  point  of  nitrogen. 

Boiling  point  of  argon. 


Liquid  air,  according  to  proportion  of  oxygen. 


Oxygen  boils. 
Alcohol  freezes. 


288  APPENDIX 

170°A.  Critical  temperature  of  air  above  which  it  cannot  be  lique- 
fied. 

181°A.  Lowest  air  temperature  recorded  by  means  of  sounding 
balloons  (at  Batavia,  Java,  November  5,  1913,  at  an 
elevation  of  17,000  meters) .  Above  this  the  temperature 
rose. 

194°A.  Obtained  by  Rotch  with  sounding  balloon,  June  25,  1905, 
at  an  elevation  of  14,800  meters. 

195°A.  Carbon  dioxide  boils.     Plant  life  ceases. 

2 13° A.  Lowest  temperature  recorded  by  Scott  in  his  expedition 
to  the  south  pole. 

233°A.  Mercury  freezes. 

273°A.  Water  freezes. 

277°A.  Water  at  maximum  density. 

287°A.  Earth's  mean  temperature. 

351°A.  Alcohol  boils. 

373°A.  Water  boils. 

504°A.  Tin  melts. 

600°A.  Lead  melts. 
1234°A.  Silver  melts. 
1283°A,  Temperature  of  boiling  lava. 
1336°A.  Gold  melts. 
1356°A.  Copper  melts. 
1698°A.  Invar  melts. 
1773°A.  Cast  iron  melts. 
1800°A.  Iron  melts. 

3723°A.  Temperature  of  the  electric  arc. 
4073°A.  Temperature  of  positive  crater  of  arc. 
6000°A.  Sun's  temperature. 
7800°A.  True  arc  high  pressure. 


APPENDIX 


289 


F.    C.     ABSOLUTE     NEW 

140- 

—  60 

; 

20 

~ 

—330 

10 

f30- 

5" 

• 

—1200 

E 

7 

90 

120- 

r—  SO 

: 

80 

tin 

E, 

1-320 

70 

t*n 

il  Q 

=-« 

r 

ov 
50 

100- 

- 

'—310 

40 

n» 

30 

90- 

E 

:r 

20 
in 

60  ~ 

E 

-300 

IU 

—1100 

— 

-      90 

70  -i 

-   p* 

~ 

80 

r~  f" 

-      70 

60-^ 

—• 

'-290 

60 

~ 

50 

SO-_ 

~         JQ 

" 

-      40 

- 

I  po/> 

30 

W-_ 

Z. 

_    tOv 

20 

- 

- 

10 

30  -_ 

~o 

—1000 

^ 

r-270 

-      90 

20-^ 

~ 

-    so 

E 

— 

70 

10-. 

r/e 

—260 

60 

-      50 

- 

— 

•     40 

- 

—  20 

-      30 

10  -_ 

— 

—250 

-      20 

~ 

10 

- 

E 

—  900 

: 

E—  30 

-    so 

30  -_ 

E. 

—240 

-    so 

•      70 

- 

— 

60 

*0-E 

E-4d 

-      50 

I 

— 

—230 

40 

10  -i 

ZT 

-      30 

- 

""      ^/\ 

20 

60  -i 

E~ 

—220 

10 

70  -J 

E" 

—  800 

•    so 

1 

""—-  L       (yQ 

-    so 

SO  -_ 

E 

—210 

-       70 

I 

— 

-      60 

90  -_ 

5 

•      50 

~ 

=—70 

-      40 

/00-E 

- 

-200 

-      30 

- 

: 

-      20 

110  — 

—  <Sd 

•       10 

120  -_ 

~ 

-190 

—  700 

-      30 

I 

— 

80 

130  -_ 

E—  ^o 

70 

~ 

: 

-180 

60 

140  -, 

7                  > 

<£                      > 

650 

439  -»-    273 


-0 


In  Fig.  114  the  three  well-known 
temperature  scales  are  charted  side 
by  side  for  quick  comparison. 
There  is  also  given  a  fourth  scale, 
marked  New,  which  is  a  variation 
of  the  Absolute  Scale,  the  zero  being 
the  Absolute  zero,  and  the  1000 
degree  mark  the  temperature  of 
melting  ice  or  273°A.  Like  the 
Absolute  Scale,  this  has  no  minus 
signs. 


FIG.  114.    TEMPERATURE  SCALES 
20 


290 


APPENDIX 


TABLE  11.     BEAUFORT  WIND  SCALE 


Beaufort 
number 

Description 

Pressure 
in  kbs. 

British  Meteor. 
Office  values 
(Miles  /hour) 

Weather 
Bureau  values 
(Miles  /hour) 

0  

Calm 

.0 

less  than  1 

0-3 

1  

Light  air 

.01 

1-3 

3-8 

2  

Light  breeze 

.04 

4-7 

8-13 

3  

Gentle  breeze 

.13 

8-12 

13-18 

4  

Moderate  breeze 

.32 

13-18 

18-23 

5  

Fresh  breeze 

.62 

19-24 

23-28 

6  

Strong  breeze 

1.1 

25-31 

28-34 

7  

Moderate  gale 

1.7 

32-38 

34  40 

8  

Fresh  gale 

2.6 

39-46 

40-48 

9  

Strong  gale 

3.7 

47-54 

48-50 

10  

Whole  gale 

5.0 

55-63 

56-65  ' 

11  

Storm 

6.7 

64-75 

65-75 

12  

Hurricane 

8. 

75- 

75- 

This  is  a  scale  adopted  by  Admiral  Beaufort  and  used  in 
connection   with   sailing   vessels   of   the   last   century.     It   is 
unfortunate  that  it  was  ever  seriously  used  by 
meteorologists.     The     most     recent     equivalent 
values  are,   as  given  by  Galitzin,   in  accordance 
with  the  decisions  of  the  International  Meteorological  Com- 
mittee at  the  Rome  meeting  in  1913. 


Force  Meters 

Beaufort  scale  per  second 

0 0 

1 1 

2 2-3 

3 4-5 

4 6-8 

5 9-10 

6..  11-13 


Force  Meters 

Beaufort  scale    per  second 


10.  . 

11. . 

12.. 


14-17 
18-20 
21-24 
25-28 
29-33 
34  and  over 


For  the  c.g.s.  units  the  relations  are,  if  /  is  the  force  upon 
a  disk  one  square  meter  in  area  facing  the  wind,  and  v  the 
velocity  in  meters  per  second, 


and  if  B  is  the  Beaufort  number,  then 

/=4.7853 
and  fl  =  0.26Vl^T 


APPENDIX 


291 


TABLE  12.     GRAMS  OP  AQUEOUS  VAPOR  PER  KILOGRAM  OF  AIR 
AND  VAPOR  AT  SATURATION 


Temperature 
Absolute 

Vapor  pressure 

Atmospheric  pressure 

1,000  kbs. 

800  kbs. 

400  kbs. 

273 

6.1  kbs.            3.5  grs. 

4.7  grs. 

9.  5  grs. 

274 

6.6 

3.8 

5.1 

10.2 

275 

7.1 

4.1 

5.5 

11.0 

276 

7.6 

4.4 

5.8 

11.8 

277 

8.1 

4.7 

6.3 

12.7 

278 

8.6 

5.0 

6.7 

13.6 

279 

9.3 

5.4 

7.2 

280 

10.0 

5.8 

7.7 

281 

10.7 

6.2 

8.3 

.... 

282 

11.5 

6.7 

8.9 

283 

12.2 

7.1 

9.5 

284 

13.1 

7.6 

10.2 

285 

13.9 

8.1 

10.9 

286 

14.9 

8.7 

11.6 

287 

15.9 

9.2 

12.4 

288 

17.0 

9.9 

13.2 

289 

18.0 

10.6 

14.1 

290 

19.0 

11.2 

15.0 

291 

20.3 

12.0 

16.0 

292 

21.6 

12.8 

17.0 

293                  23  .  2 

13.6 

18.0 

294                 24.6 

14.5 

19.4 

295 

26.0                 15.0 

21.0 

296 

28.0 

16.0 

22.0 

297 

29.0 

17.0 

23.0 

298 

32.0 

18.0 

25.0 

299 

33.0 

20.0 

300 

35.0 

21.0 

301 

37.0 

22.0 

.... 

302 

40.0 

23.0 

303 

42.0 

25.0 

304 

44.0 

26.0 

305 

47.0 

28.0 

.... 

292 


APPENDIX 


TABLE  13.     GRAMS  OF  AQUEOUS  VAPOR  PER  CUBIC  METER 
AT  SATURATION 


°A. 

grams 

°A. 

grams 

°A. 

grams 

273 

4.835 

283 

9.330 

293 

17.118 

274 

5.176 

284 

9.935 

294 

18.143 

275 

5.538 

285 

10.574 

295 

19.222 

276 

5.922 

286 

11.249 

296 

20.355 

277 

6.330 

287 

11.961 

297 

21.546 

278 

6.761 

288 

12.712 

298 

22.796 

279 

7.219 

289 

13  .  505 

299 

24.109 

280 

7.703 

290 

14.339 

300 

25  .  487 

281 

8.215 

291 

15.218 

305 

33  .  449 

282 

8.757 

292 

16.144 

310 

43  .  465 

THE    INDEX 


Abbe,  C.,  Jr.,  cited,  230n 

Abbott,  C.  G.:  cited,  In,  18,  164,  277,  278; 
solar  radiation  constant,  275 

Abercromby,  Ralph:  cloud  nomenclature, 
112,  113;  quoted,  118;  cited,  123 

Absolute  scale:  see  Scale 

Absolute   temperature:    see  Temperature 

Absolute  zero,  34 

Absorption  and  solar  radiation,  275 

Academy  of  Sciences,  Paris,  balloon 
ascensions  of  1804  under  auspices  of, 
10;  ascensions  of  1875,  10 

Acceleration:  normal,  of  gravity,  [Table] 
28;  normal,  symbol  for,  83;  pro- 
duced by  slight  gradient,  58;  unit 
of,  26,  28 

Adiabat:  dry,  144,  145;  condition  rare,  44 

Adiabatic  diagram:  [Fig.  57]  141;  applica- 
tion of,  by  Neuhoff,  144;  methods  of 
making,  142;  methods  of  using,  143 

Adiabatic  equilibrium,   137 

"Advective,"  46 

"Aer,"  30 

Aerological  Congress,  at  Monaco,  29; 
at  Vienna,  31;  see  also  the  interna- 
tional associations 

Aerological  research  expeditions,  15,  17 

"Aeronaut,"  105 

Aeronautics:  international  cooperation 
of  observatories,  13,  14;  scientifc, 
International  committee  for,  13,  47 

Aeronauts,  congresses  of,  13 

African  dust  in  southern  Europe,  156 

Air:  and  vapor,  mixing  equations,  143; 
ascending,  of  cyclones,  54;  at  high 
level  as  medium  of  radiation,  261; 
compressibility  of,  law  of,  6;  charac- 
teristic equation  for,  40;  cold,  down- 
draft  of,  in  thunderstorm,  178; 
density,  fluctuations  in,  188;  de- 
scending, of  anticyclones,  54;  drain- 
age, local,  259;  dry,  137,  140; 
dynamical  heating  and  cooling,  41; 
downrush  of,  velocity  of,  182;  ex- 
plorations of,  3,  7;  friction  and  elec- 
trification, 170;  "holes  in,"  107; 
hot,  of  cities,  as  carrier  of  dust,  160; 
humid,  rapid  convection  of,  and 
thundercloud,  174;  intensity  of  ver- 
tical movement  determined  by  mix- 
ing ratio,  268;  liquid,  temperature 
for,  287;  low  temperature  obtained 
by  Rotch,  288;  lower,  stratification 
of,  107;  lowest,  temperature  re- 
corded, 288;  masses,  mixtures  of, 
146;  moist,  and  condensed  vapor, 
difference  between,  26;  moist,  stages 
accompanying  changes  in,  140; 
molecular  weight  of,  24;  necessity 


of  nuclei  in,  154,  157;  nucleation, 
measured  by  corona  method  of 
Barus,  150;  number  of  dust  particles 
in,  [Aitken's  Table]  160;  path  of,  and 
sea  temperature  during  fog  off 
Newfoundland,  [Fig.  61]  149;  "pock- 
ets" in,  107;  potentially  cold,  descent 
of,  as  cooling  agent  in  thunder- 
storms, 176;  samples,  collection  of, 
from  balloon,  10;  saturated,  condi- 
tions of  equilibrium  for,  138;  stag- 
nant, and  frost,  260;  stratification, 
sleet  showing  conditions  of,  232; 
sulphur  dioxide  in,  162;  surface,  flow 
of,  and  frost  forecast,  259;  tempera- 
ture in  ice  storms,  231;  temperature 
inversions  by  vertical  movements  in, 
259;  transparency  of,  and  dust,  160; 
unsaturated,  140;  unsaturated,  dust 
in,  and  humidity,  160;  velocity,  near 
center  of  tornado  vortex,  93;  veloc- 
ity, toward  equator,  and  toward 
poles,  57;  vitiated,  and  disease,  155; 
wave  motion  in,  123;  with  vapor  and 
fluid  water,  140;  with  vapor  and  ice, 
140;  with  vapor,  liquid,  and  ice,  140: 
see  also  Atmosphere;  Currents;  Cy- 
clones; Winds 

currents:  ascending  and  descending, 
107;  deviations  due  to  deflective 
effect  of,  57;  in  thunderstorm,  171; 
mingling,  linear  momentum  in,  182; 
strength  of,  varies  with  height,  107; 
strongest,  location  of,  107;  under- 
currents, cold,  direction  of,  123;  upper 
air,  123;  vertical  velocity  of,  and 
rainfall,  171;  westerly  component 
of,  80 

flow:  260;  change  in  velocity  and  direc- 
tion of,  as  a  forecast,  264;  in  moun- 
tainous regions,  103 

motion:  center  of  movement  of,  78; 
cloud  motion  not  exponent  of,  125, 
151;  deflection  of,  and  pressure  dis- 
tribution, 56;  not  wholly  controlling 
cloud  motion,  125,  151;  records  of, 
difficult,  151 

movement:  and  absolute,  relative  and 
reversed  velocities,  55;  and  terres- 
trial rotation,  54;  deflective  effect  of 
earth,  55;  moderate,  and  high  summit 
temperature,  264;  relative  changes 
in,  55;  relative  to  rotation,  61 

specific  heat:  37;  at  constant  pressure, 
34,  [formula]  43;  at  constant  volume, 
34 

streams:  great,  constancy  of,  55;  strong, 

and  frost  formation,  259 
Air  pump  of  Otto  von  Guericke,  5 


293 


294 


THE  INDEX 


Aitken,  John:  dust  counter,  157;  [Fig.  63] 
158;  koniscope,  161;  observations  on 
dust  in  atmosphere,  148,  154;  on 
dust  as  nuclei  of  condensation,  156, 

o       162 

Akerblom,  Filip,  cited,  112 

Alberta  storms,  87 

Alcohol:  boiling  point  of,  288;  freezing 
point  for,  287 

Aldrich,  L.  B.,  cited,  18,  278 

Aleutian  infrabar,  66,  67,  69 

Alexander  the  Great:  his  information  of 
monsoons,  first  in  Greece,  2,  10 In 

Alexander,  P.  Y.,  Amazon  expedition,  17 

Algeria,  Bassour  Observatory,  solar-con- 
stant records  of,  276 

Allobar,  90 

Alpha  radiation:  see  Atmosphere,  upper 

Altitude,  method  showing  diminution  of 
temperature  with,  142 

Alto-cumulus:  115,  123;  [Fig.  37]  114; 
[Fig.  41]  122;  indicating  rain  proba- 
bility, 130 

Alto-stratus:  115,  123;  [Figs,  38,  39]  116; 
[Fig.  40]  120;  indicating  rain  proba- 
bility, 129 

Amazon,  aerological  expedition  to,    17 

American  Association  for  the  Advance- 
ment of  Science,  explorations  of 
Captain  Wilkes  published  by,  97 

American  Institute  of  Electrical  Engi- 
neers: adoption  of  standard  of  value 
of  horse-power,  27;  De  Blois'  investi- 
gations on  character  of  lightning 
discharge,  189w 

American  Meteor  Society  of  McCormick 
Observatory,  established,  20 

American  System,  New,  of  pressure 
units,  [Table]  30 

American  University,  Washington,  D.  C.: 
measurements  of  diffuse  sky  radia- 
tion, 278 

Ames,  Prof.  J.  S.,  quoted,  33,  41 

Ammonia:  from  lightning  discharge,  198; 
in  air,  6« 

Amundsen,  Captain  Roald,  cited,  15 

Anallpbar,  90 

Anaximander  of  Ionia,  cited,  1 

Anderson,  J.  A.,  cited,  202 

Anemometers,  Robinson,  record  of,  105 

Angot,  Alfred,  cited,  218 

Angles,  decimalization  of,  not  adopted, 
27 

Angstrom,  K.,  cited,  278 

Angular  momentum,  constancy,  law  of,  57 

Angular  radius,  symbol  for,  83 

Angular  velocity  of  earth's  rotation,  56, 
57;  formula,  57;  of  point  on  earth's 
surface,  62;  symbol  for,  84:  see 
Earth,  rotation  of 

Antarctic  currents,  cold,  70;  expedition, 
German,  17 

Antarctica,  interior  of,  flow  of  ice  from, 
71;  temperature  of  land  mass,  70 

Anthelion,  165 


Anticyclones:  77-86;  and  ice  storm,  234; 
cloud  distribution  in,  [Fig.  42]  124; 
control  of  weather  in  United  States, 
69;  curvature  gradient  and  rotation 
gradient  in,  64;  descending  air  of,  54; 
level  of  maximum  rotation,  63;  light 
winds  in  central  region  of,  84; 
northern,  dominating  southern  cy- 
clones, 237;  pressure  distribution  in, 
[Fig.  26]  81;  relation  of  cloud  mo- 
tions to,  110;  separation  of  isobars 
in  inner  regions  of,  63;  temperature 
distribution  in,  62,  [Fig.  25]  80; 
temperatures  of  front  and  back  of, 
78;  warm-centered  and  cold-cen- 
tered, 78,  79;  wind  distribution  in 
[Fig.  24]  79:  see  also  Cyclones 

Anti-trades:  97,  100;  height  of,  51:  see 
also  Trades 

Apalachicola  storm  of  1915,  257 

Aqueous  vapor:  and  vapor  of  saturation, 
[Table  12]  291;  at  saturation,  [Table 
13]  292;  condensation  of,  137;  dis- 
sipation of,  152 

Arc:  circumzenithal,  165;  electric  tem- 
perature of,  288;  positive  crater, 
temperature  of,  288;  true,  high 
pressure,  temperature  of,  288 

Archibald,  Douglas,  cited,  8 

Arctic  Ocean  currents,  70,  73 

Area,  unit  of,  26 

Argentine-counter  current,  72 

Argon:  boiling  point  of,  287;  and  nitro- 
gen, percentage  of,  in  air,  Qn;  per- 
centage in  air,  22;  separated  from 
atmospheric  nitrogen,  7 

Aristotle,  cited,  2 

Ascents,  international,  for  upper  air 
investigations,  13 

Askesian  Society:  published  Luke  How- 
ard's cloud  classification,  111 

Assmann,  Dr.  Richard,  cited,  10.  11,  15,46 

Astro-meteorology,  1 

Astronomical  Society  of  Antwerp:  estab- 
lished Le  Bureau  Central  Meteo- 
rique,  20 

Astrophysical  Observatory  of  the  Smith- 
sonian Institution:  measurements  of 
solar  radiation,  18;  report  of  1913  on 
radiation  and  sunspot  numbers,  276 

Atlantic,  balloon  record  over,  148 

Atlantic  coast  types  of  storms:  see  storms, 
types  of 

Atlas,  Bartholomew's,  217;  international, 
of  cloud  forms,  113 

"Atmospheric  billows,"  151 

Atmospheric  electricity,  167-204 

Atmosphere:  absolute,  27,  28;  and  earth's 
surface,  relative  motions  of,  60;  and 
solar  radiation,  275;  approximate 
weight  of,  32;  bacteria  in,  154; 
chemical  composition  of,  6;  circula- 
tion of,  54-64;  classification  of, accord- 
ing to  adiabatic  gradient,  139;  con- 
ductivity of,  199;  constituents  of, 


THE   INDEX 


295 


7;  descent  of,  in  rainfall  and  thunder- 
storm, 178;  determining  the  height 
of,  19,  20;  distribution  of  gases  in,  21, 
[Fig.  8]  23;  dust  content  of,  155; 
earliest  investigations  of,  3;  early 
studies  of  pressure  of,  3,  5,  6;  effect 
of  earth's  rotation  on,  54;  effect  of 
ocean  currents  on  circulation  of,  70; 
electrical  phenomena  of,  incidents  of 
storm,  168;  electricity,  origin  of,  168; 
equations  of  motion  of,  54;  foreign 
matter  in,  154;  gases  in,  distribution 
of,  22;  gases  in,  formula  for  densities 
of,  in  molecular  weight,  24;  homo- 
geneous, non-existent,  21;  horizontal 
circulation,  83;  in  early  treatises  on 
meteorology,  21;  isothermic,  condi- 
tions for,  139;  lowest  temperatures 
reached  in,  48,  [Table]  49;  lower, 
mechanism  of,  78;  mass  of,  30; 
nucleation,  161;  persistence  of  dis- 
tribution, 85;  relative  motions  of, 
and  motions  of  earth's  surface,  60; 
salt  content,  164;  slow-moving  cur- 
rents of  lower  strata,  259;  sources  of 
dust,  155;  structure,  types  of,  106; 
thermodynamics  of,  37-45,  146; 
transmission  and  sunlight,  162;  vol- 
ume, 32;  water  vapor  of,  108-136; 
waves  in,  clouds  as  crests  of,  125; 
west-east  drift,  63;  see  also  Air;  Cur- 
rents; Cyclones;  Winds 
motion:  relative  to  earth's  surface,  60; 
Shaw's  five  laws,  82;  see  also  Air 
Movements;  Circulations 
pressure:  at  right  and  left  of  prevailing 
winds,  63;  hydrostatic,  first  knowl- 
edge of,  3;  units  of,  27 
upper:  alpha  radiation  in,  278;  ioniza- 
tion  showing  radiation  in,  278; 
radio-active  cosmical  dust  in,  278; 
radio-active  radiation  in,  278 

Aurora,  19,  278;  and  ionization  of  upper 
atmosphere,  278;  and  radio-active 
radiation,  278;  average  height  of 
bottom  of,  278;  cosmic  rays  of, 
groups,  278;  physical  nature  in 
different  manifestations  of,  278; 
international  symbol  for,  31 

Auroral  arc,  measurements  of,  19 

Australian-counter  current,  72 

Australian  hyperbar,  66 

Australian  soundings,  self-recording  theo- 
dolites, used  in,  18 

Autumn  sun,  heat  of,  274 

"Aviator,"  105 

Avogadro,  Amadeo,  cited,  41;  Avogadro's 
law,  41 

Azores  hyperbar,  66,  98 

Bacon,  M.  L.,  cited,  93 
Bacteria  in  atmosphere,  7,  154 
Baguio,  P.  I.:  exceptional  rainfall  at,  214; 
heaviest  recorded  rainfall,  [Fig.  81]  215 
Baguio  type  of  cloud  as  forecast,  125 


Ball  lightning,  192,  193 

Ballons-sondes:  11,  85;  filling,  [Fig.  4] 
14;  launching,  [Fig.  3]  13;  observa- 
tions with,  13 

Balloons:  7,  9;  at  great  altitudes,  records 
of,  17;  courses  taken  by,  in  May, 
1906,  [Fig.  5]  16;  extreme  elevations 
reached  by,  19;  first  meteorological 
survey  by,  9;  free,  observations  with, 
13;  in  Antarctic  expedition,  15;  in 
Danish  expedition  to  Greenland,  15; 
international  ascents,  13;  manned 
observations  with,  13;  record  over 
Atlantic,  148;  registering,  African 
expedition,  15;  registering,  first 
ascension  of,  in  America,  [Fig.  2]  12; 
sounding,  11;  observations  with,  13; 
meteorological  investigations  by 
Lindenberg  Observatory,  15,  49; 
pilot,  16,  18,  85;  pilot  ascensions  at 
Blue  Hill  Observatory,  18;  records  at 
Pavia  Observatory,  18;  tests  by,  107; 
used  at  Blue  Hill,  [Fig.  7]  19 

Bars,  30;  and  billows  in  frost  conditions, 
268 

Barker,  David  W.,  cited,  112 

Barkow,  Dr.  E.,  meteorological  records  on 
German  Antarctic  Expedition.  17 

Barometer,  3,  5;  change,  in  thunder- 
storm, 181;  "  Torricelli's  tube,"  3; 
water-,  of  von  Guericke,  5;  pressure 
at  time  of  Galveston  flood,  252; 
factors  contributing  to  increase  of, 
during  thunderstorm,  183;  Houston, 
Texas,  during  storm  of  August,  1915, 
[Fig.  100]  253 

Barometric  gradient  and  wind,  relation 
between,  63;  balance  of,  by  velocity, 
63 

Barometric  tendency,  defined  by  the 
International  Commission,  89 

Baroscope:  see  Barometer 

Barral,  J.  A.,  cited,  10 

Bartholomew's,  Atlas,  217 

Barus,  Carl,  cited,  154,  157w,  161; 
corona  method  of  numbering  nuclea- 
tion of  air,  150 

Bassour  Observatory,  Algeria:  observa- 
tions of  fluctuations  in  solar-constant 
values,  276 

Batavia  Observatory,  Java,  lowest  upper 
air  temperature  recorded  by,  18; 
records  of  temperature  at  strato- 
sphere, 49,  51 

Bauer,  L.  A.,  cited,  54,  55 

Beals,  E.  A.,  cited,  6Qn 

Beaufort  wind  scale,  290;  equivalent 
values  for,  290;  formula  for,  290, 
[Table  11]  290 

Bell,  Alexander  Graham,  cited,  9 

Benndorf  electromometer,  register  of,  at 
Simla,  169 

Bentley,  Wilson  A.,  observations  of  rain- 
drops, 206;  study  of  snow  crystals, 
221;  cited,  243,  244w 


296 


THE   INDEX 


Berghaus,  Heinrich,  cited,  217 

Bering  current,  73 

Bermuda  hyperbar,  65,  69;  effect  on 
Atlantic  coast,  69;  effect  of  Labrador 
current  on,  69 

Berson,  Dr.  Arthur,  cited,  9,  11 

Besancon,  Georges,  and  sounding  bal- 
loons, 11 

Besson,  Louis,  cited,  164 

Bezola,  W.  von,  cited,  260,  267;  discus- 
sion of  precipitation,  146,  150; 
abstract  of  tables  of  precipitation, 
147;  discussion  of  fog,  147 

Bigelow,  Prof.  F.  H.,  cited,  44rc;  77,  78, 
93,  94;  report  on  international  cloud 
observations,  110;  diagrams  of  cloud 
types,  [Fig.  40]  120,  121,  [Fig.  41] 
122;  diagrams  of  distribution  of 
cloud  types,  [Fig.  40]  120;  [Fig.  41] 
122 

Billows  and  bars  in  frost  conditions,  268 

Biot,  J.  B.,  cited,  10 

"Bishop's  ring,"  from  volcanic  dust,  163 

Bixio,  J.  A.,  cited,  10 

Bjerknes,  Prof.  V.,  cited,  30;  table  of 
adiabatic  gradients,  138;  classifica- 
tion of  variations  of  adiabatic 
gradients  by  atmospheres,  139; 
classification  according  to  dynamic 
heights,  139;  quoted,  220;  table  of 
adiabatic  gradients,  138 

Black,  W.  M.,  cited,  6;  observations  of 
dust  deposit,  155 

Blair,  W.  R.,  cited,  17 

Blanchard,  J.  P.,  aeronaut,  9 

Blanford  H.  F.,  cited,  216w 

Blizzard,  Great,  of  New  York,  89w 

Blue  Hill  Observatory:  Campbell-Stokes 
sunshine  recorder,  use  of,  132;  elec- 
tric potential  values  obtained,  199; 
electric  potential,  observations  of, 
by  McAdie,  8,  8n;  halos  and  pre- 
cipitation, studies  by  Palmer,  164; 
kite  flight,  and  discharge  without 
lightning  and  thunder,  167;  new 
units  adopted  by,  30;  ombroscope 
used  for  registering  rainfall,  211; 
pilot  balloon  ascensions,  first  in  U.  S., 
18;  Pole-star  recorder  at,  133;  rain- 
fall record  of,  211;  wind  charts  for 
aeronauts  issued  by,  105;  wind  cur- 
rents, ascensions  to  study,  18 
clouds:  average  height  of  cirrus,  52; 
cirrus  movements,  observation  of, 
131;  Clayton's  law  demonstrated  in 
observations,  81w;  cumuli,  tops  of, 
colder  than  air  at  same  level,  153; 
measurements,  110;  records,  134 
Ice  Storms:  Brooks'  discussion  of 
temperatures,  131;  diagram  of  pre- 
cipitation and  temperature  during, 
233,  [Fig.  88]  234;  lowest  air  tem- 
perature recorded,  during  rainfall  at, 
232;  northeasterly  type  of,  235, 
[Fig.  89]  235,  236;  northwesterly 


type  of,  237;  observations  of  cirrus 
at,  52,  131;  photographs  of  by  L.  A. 
Wells,  [Fig.  91]  240,  [Fig.  92]  241, 
[Fig.  93]  242,  [Fig.  94]  243;  records  of 
balloon  explorations  by,  105;  records, 
unpublished,  of  details  of,  238n; 
thermograph  curves,  showing  sudden 
changes  during,  239,  [Fig.  90]  239, 
240;  use  of  term  sleet,  233n 
Inversions:  and  frosts,  [Fig.  107]  265; 
records,  261,  262,  [Fig.  106]  263; 
saturation  deficit  recorder,  [Fig.  110] 
268;  types  of,  [Fig.  105]  261 

Blue  of  the  sky,  163 

Bolides,  20 

Bolton,  H.  C.,  cited,  3w 

Bonacina,  L.  C.  W.,  cited,  230 

Bora,  of  the  Adriatic,  105 

Boreal  winds,  104 

de  Bort:  see  Teisserenc  de  Bort 

Bowie,  Edward  H.,  formula  of.  28;  storm 
types  of,  87;  rules  of  12-hour  pressure 
change,  88;  cited,  258 

Boyle,  Robert,  cited,  3,  5,  6;  book  on  air, 
5;  law  of  Boyle,  6,  37 

Boyle  and  Mariotte's  law,  6,  39 

Boyle-Gay-Lussac  law,  6w 

Braak,  cited,  50;  quoted,  51;  cited,  52 

Bravais,  halo  of,  165 

"Brave  west  winds,"  100 

Breaking-drop  theory  of  electrical  separ- 
ation, 171 

Breeze:  land  and  sea;  mountain  and 
valley,  98 

Brickfielders  (hot  north  winds),  104 

British  Association  for  the  Advancement 
of  Science:  aerial  explorations  di- 
rected by,  7,  10;  report  on  balloon 
investigations,  7 

British  Rainfall  Organization,  and  use  of 
British  rain  gauge,  209 

British  thermal  unit,  270;  absorbed  in 
changing  ice  to  water,  and  water  to 
steam,  271 

"Brocken  specter,"  166 

Brooks,  C.  F. :  study  of  snow  distribution 
and  ice  storms,  224,  229,  231,  235n 

Bruckner  period  of  tree  growth,  216 

Brush  discharges:  wandering,  192;  on 
thunderstorm  cloud,  193 

Buchan,  Dr.  Alexander,  cited,  63,  66,  217 

Buran,  of  Russia,  105 

Bureau  Central  Meteorique  le,  estab- 
lished by  Astronomical  Society  of 
Antwerp,  20 

Bureau  of  Standards,  bulletin  on  light- 
ning rods,  202 

Burster,  Southerly,  of  Australia,  105 

Buys-Ballot,  C.  H.  D.:  Buys-Ballot's 
law,  63 

California :  precipitation  in,  212,214;  snow 

data  from,  225 
California  current,  74 
California  hyperbar,  65 


THE   INDEX 


297 


Callendar  pyrheliometer,  277 

Calories:  small,  34,  42;  required  to  raise 
temperature  of  dry  air,  137 

Campbell-Stokes  sunshine  recorder,    132 

Canada,  Meteorological  Service  of,  bal- 
loon records,  19 

Cap-clouds,  181 

Cape  Horn  current,  72 

Capper,  James,  cited,  77 

Capus,  G.,  cited,  199 

Carbon  dioxide:  boiling  point  of,  288; 
in  air,  Qn;  molecular  weight,  24;  per- 
centage in  air,  22 

Carbon  monoxide,  molecular  weight,  24 

Carnegie  Institution,  Department  of 
Terrestrial  Magnetism:  radioactive 
content  of  ocean,  201 

Castelli,  Benedetto,  cited,  207 

Cast  iron,  melting  point,  288 

Cave,  J.  C.  P.:  cited,  7,  15,  81;  observa- 
tions by,  106,  107;  models,  106 

Cavendish,  Henry,  cited,  6,  9 

Celsius  scale,  35 

Centibar,  29 

Centrifugal  force:  in  upper  and  lower 
strata,  62;  at  level  of  maximum 
drift,  63 

Centrigrade  scale,  Absolute,  35,  289 

Center  of  gyration,  78 

Centimeter,  25,  31 

Centimeter-gram-second  system,  25 

Central  counter  current,  73 

Central  storm,  87 

Ceraunoscope,  indicating,  use  of,  for 
lightning  records,  190 

"c.  g.  s."  system,  25 

Change  of  winds  near  center  of  New 
Orleans  storm  of  September,  1915. 
[Fig.  103]  256 

Channon,  J.  B.,  lines  in  spectrum  of 
lightning,  [Table]  196 

Chaplin,  Arnold,  cited,  2 

Characteristic  equation  for  air,  40 

Charge  decrease  and  rainfall  intensity, 
170;  per  unit  in  snowfall  and  in 
rainfall,  170;  positive  and  negative, 
relative,  in  snow,  170 

Charles  and  Gay-Lussac,  law  of,  39; 
formula,  40 

Charles,  J.  A.  C.,  law  of  Charles,  39 

Charts  and  maps  of  winds,  early,  97;  for 
aeronauts  and  aviators,  105 

Charting  rainfall,  220 

Chemical  effects  of  lightning,  198 

Chinook  winds,  104;  application  of 
Neuhoff's  diagram  in  observations, 
144 

Church,  Prof.  J.  E.,  Jr.,  snow  sampler, 
223;  economic  importance  of  snow 
sampler,  225 

Cigarette  smoke,  particles  in,   160 

Cincinnati's  dust  estimate,  156 

Circulation,  anticyclonic,  61;  major,  65- 
76;  minor,  77-86;  planetary,  Ferrel's 
scheme  of,  56;  within  cloud  masses, 
151 


Cirri,  false.  181 

Cirriforms,  [Clayton's  chart]  119 

Cirro-cumulus,  115,  [Fig.  40]  120;  and 
fair  weather,  130;  and  the  raindrop, 
207;  height,  [Fig.  15]  52 

Cirro-stratus,  113,  [Fig.  40]  120,  123;  and 
halo,  164;  and  rain  probability,  129; 
and  the  raindrop,  207;  height. 
[Fig.  14]  52 

Cirrus,  111,  112,  113,  [Fig.  40]  120, 
[Fig.  46]  127,  [Fig.  48]  128;  and  cold 
wave,  130;  and  forecasts  of  tem- 
perature, 127;  and  storm,  130;  and 
storm  movements,  131;  and  strato- 
sphere, 51;  bands,  121,  123,  [Fig.  49] 
129;  direction,  123;  base,  51,  52;  con- 
trolled by  temperature  gradients, 
127;  height,  [Fig.  13]  52;  movements 
controlled  by  temperature  gradients, 
127;  movements,  directions  of,  131; 
observations  at  Blue  Hill,  52,  131; 
plumes,  [Fig.  47]  128;  type  of  cloud 
as  forecast,  125;  level,  123;  cloud 
formation  in,  52;  of  radiation 
change,  53;  temperatures  at,  [Table] 
51 

"Claps"  of  thunder,  189 

Clayden,  A.  W.,  classification  of  clouds, 
110,  112 

Clayton,  H.  H.,  cited,  9,  110,  118;  classi- 
fication of  clouds  according  to  alti- 
tude, [Chart]  119;  according  to 
origin,  112;  Clayton's  law,  57,  81; 
cloud  measurements,  110,  112,  118; 
letters  for  cloud  names,  119;  observa- 
tions, 123;  diagrams  of  distribution 
of  clouds  in  cyclones  and  anticy- 
clones, [Fig.  42]  124 

Climate,  continental  and  marine,  differ- 
entiated by  Romans,  2 

Cline,  Dr.  I.  M.,  cited,  246.  253,  258 

Clos,  cited,   112 

Cloud:  altitudes,  measurement  of,  110; 
analysis  of  light  from,  10;  and  fogs, 
originating  of,  150;  annual  variation, 
53;  as  agency  of  transmission  of 
energy,  111;  as  crests  of  atmospheric 
waves,  125;  as  exponent  of  condensa- 
tion, 111;  atlas  prepared  by  Inter- 
national Committee,  113;  banners 
and  cloud  caps,  212;  billows,  123; 
cap-,  181;  caps,  and  cloud  banners, 
212;  change  maps  in  forecasting,  89; 
changes,  [Fig.  37]  114;  [Figs.  38,  39] 
116;  changes  in  nomenclature  of, 
118;  cirrus:  see  Cirrus;  cumulus: 
see  Cumulus;  difference  of  tempera- 
ture in,  174;  distribution  in  cyclones 
and  anticyclones,  [Fig.  42]  124; 
figures  indicating  density  of,  118; 
flocciform,  [Clayton's  chart]  119; 
forms,  atlas  of,  113;  fundamental 
types  of,  118;  international  instruc- 
tions for  recording,  117;  intervals, 
moonlight  record  with,  [Fig.  54]  133; 


298 


THE    INDEX 


lightning  discharge  through,  [Fig.  72] 
190;  masses,  circulation  within,  151; 
observations  at  Potsdam  Observa- 
tory, 110;  particles,  diverse  action  of, 
161;  ragged,  151;  "rank  and  file," 
151;  significance  of,  111;  squall,  179; 
studies  by  Clay  den,  110;  turbulence 
within,  151;  types,  distribution,  121; 
undulatory,  117;  unreliable  in  fore- 
casting, 125;  upper,  radiant  point  of, 
117;  upper,  observations  at  Zi-ka-wei 
Observatory,  127;  "wool  pack,"  115: 
see  also  Cyclones;  Electricity;  Rain; 
Storm;  Thunder 

classification  of:  110;  based  on  appear- 
ances and  on  origin,  111,  112;  Clay- 
ton's, according  to  altitude,  [Chart] 
119,  according  to  origin,  112; 
Hildebrandsson's  efforts  for,  113; 
Howard's,  111;  international,  113; 
Lamarck's,  111;  Ley's,  112;  Rus- 
sell's, 112w 

formations:  137;  and  decrease  in 
temperature  gradient,  53;  earliest 
knowledge  of,  108;  in  advance  of 
storm,  [Fig.  50]  130;  in  cirrus  level,  52 
height:  instrument  for  measuring, 
[Fig.  43]  125;  method  of  measure- 
ments in  1644,  21;  of  cirro-cumulus. 
[Fig.  15]  52;  of  cirro-stratus,  [Fig.  14] 
52;  of  cirrus,  [Fig.  13]  52;  plotting 
machine  for  measuring,  [Fig.  44]  126; 
levels:  [Clayton's  chart]  119:  see  also 

Cirrus;    Cumulus;    Stratosphere 
measurements:    in    determining    height 
of  atmosphere,  20;  of  height,  method 
in   1644,   21;  photography  employed 
in,  110 

motion:  and  condensation,  151;  inde- 
pendent and  drift,  151;  not  expo- 
nent of  air  motions,  125,  151;  rela- 
tion of,  to  cyclones  and  anticyclones 
Bigelow's  report  on,  110:  see  also  Cy- 
clones; Storms 

names:  Clayton's  letters  for,  119; 
international  letters  for,  113,  115, 
117 

records:   and  total   solar  eclipses,    136; 

by  S.  C.  Russell,  112,  134;  night,  134 

thunderstorm:  turbulence  in,  [Figs.  65, 

66]    175:  see  also   Electricity;   Rain; 

Storm;  Thunderstorm 

undulations:  direction  of  length  of ,  123; 

Helmholtz's     explanation     of,     125; 

wave  motions  in  air  shown  by,    123 

Cloudiness:  day,  record  of,  for  two 
months,  [Fig.  56]  135;  night,  re- 
corder. 133 

Cloudless  periods,  135;  in  the  Sierra,  136; 

Clouston,  cited,    112 

Coast  Guard  vessels,  U.  S.,  records  of  fog 
formation  and  dissipation,  148 

Coast  Survey,  U.  S.,  use  of  term  " grav- 
ity, "|28w 

Coffin,  J.  H.,  cited,  63;  work  on  winds, '97 


Cold  waves:  104,  259;  and  cirrus,   130 

Color:  depth  of,  of  atmosphere,  indicating 
number  of  dust  particles,  161 ;  indica- 
tions of  koniscope,  [Aitken's  table] 
161;  of  atmosphere,  indicating  size  of 
particles,  161;  of  halos,  164 

Colorado:  data  on  snow  measure  from, 
225;  type  of  storm,  87 

Combustion  and  atmospheric  dust,   155 

Compression:  adiabatic,  145;  and  kinetic 
energy,  41;  effect  on  cloud  motion, 
151 

Condensation:  137-153;  affected  by  for- 
eign matter,  154;  by  electric  dis- 
charge, 148;  by  mixture,  various 
views  of,  146;  cloudy,  Aitken's 
studies  in,  156;  conditions  present, 
140;  dust  as  nuclei  for,  7,  156;  effect 
on  cloud  motion,  151;  forms  of  direct, 
230w;  givre  from,  244;  intensity  of, 
on  slopes,  212;  latent  heat  of,  at  high 
level,  185;  minimum  elevations  for, 
212;  of  vapor  on  mountain  slopes. 
[Table  by  Pockels]  212;  power  of 
dust  particles,  160;  precondition  of, 
157;  rate  of  cooling  decreased  by, 
140;  with  muclei,  148 

Conduction,  heat  added  or  lost  by,   137 

Conductivity:  of  atmosphere,  199;  speci- 
fic, observations  of,  201;  temperature 
and  pressure,  201 

Conservation  of  energy,  principle  of,  42 

Constancy  of  angular  momentum,  law  of, 
57: 

Constancy  of  density,  law  of,  57 

Constant  pressure:  specific  heat  of  air 
under,  43;  formula,  43;  variable,  44 

Constant,  solar:  18;  values,  fluctuations 
in,  276 

Constant  volume:  percentage  of  gases  in 
region  of  constant  temperature,  23;- 
specific  heat  of  air  under,  43;  ther- 
mometer, 36 

Continental  types,  storms  of,  87 

Continental  winds,  98 

Controls,  solar  and  cyclonic,  struggle 
between,  69 

Convection:  direction  in  stratosphere,  46; 
heat  added  or  lost  by,  137;  hori- 
zontal, and  interchange  of  heat  as 
prime  movers  of  storms,  79;  law  of, 
82;  law  of  the  limit  of,  83;  limits,  24; 
of  gases  in  regions  of  temperature 
changes,  23;  phenomena  of,  in  spring 
and  fall,  69;  pressure  distribution  at 
surface  modified  by,  86;  vertical, 
and  condensation  not  prime  movers 
of  storms,  79;  vertical,  check  to 
horizontal  flow,  182 
currents:  effect  of,  due  to  insolation  at 
valley  level,  264;  extent  of,  51;  loca- 
tion of,  in  storm,  186;  strength,  and 
size  of  hailstones,  185 

"Convective,"  46 

Convective  temperature  equilibrium,    50 


THE   INDEX 


299 


Conversion:  feet  into  meters,  [Table  2] 
279;  inches  into  millimeters,  [Table  1] 
279;  miles  into  kilometers,  [Tables 
3,  4]  280;  millimeters  to  kilobars, 
[Table  8]  284;  minus  temperatures, 
[Table  10]  286,  287;  temperatures, 
[Table  9]  285;  velocities,  [Table  6] 
281,  282;  scale,  convenient  [Fig  9], 
35 

factors:  for  units,  25,  26;  of  dynamical 
and  thermal  units,  34;  of  emissivity, 
38;  of  Joule's  equivalent,  38;  of 
latent  heat,  39 

table:  for  frost  work,  270;  pressure, 
inches  to  kilobars,  283,  284 

Cooling:  adiabatic,  offset  by  heat  of 
condensation,  138;  adiabatic  rate  of, 
44,  137;  degree  of,  in  evaporation, 
152;  for  precipitation,  compared 
with  mixing,  148;  from  retarded  cur- 
rents, 264 

Copper,  melting  point  of,  288 

Coriolis,  G.,  cited,  55,  56;  Coriolis'  the- 
orem of  earth's  deflective  effect,  55, 
59,  60 

Coronas:  and  halos,  164;  lunar  and  solar, 
165;  lunar,  international  symbol  for, 
31;  method  of  Barus,  of  numbering 
nuclei  in  air,  150;  solar,  international 
symbol  for,  31 

Coronium  indicated  in  spectrum  of  solar 
corona,  7 

Cosmic  rays  of  aurora,  groups  of,  278 

Cosmical  dust,  radio-active,  in  upper 
atmosphere,  278 

Counter-currents  underrunning  drift  of 
cyclones,  78 

Counter-trades,  100 

Covert,  R.  N.,  standardized  value  for 
density  of  mercury,  284n;  for  grav- 
ity, 284w 

Cox,  Lieut.  F.  A.  D.,  R.  N.,  description 
of  waterspouts,  94 

Coxwell,  H.  T.,  cited,  10 

Crater,  positive,  of  arc,  temperature  of, 
288 

Crystals,  formation  of,  222 

Crystallization  of  snow,  221 

Cumuliforms,  [Clayton's  chart]  119 

Cumulo-nimbus,  117,  121,  [Fig.  41]  122; 
vortices  at  edge  of,  179 

Cumulus,  107,  111,  112,  115,  [Fig.  41]  122, 
171;  and  the  raindrop,  207;  behavior 
of,  near  stratosphere,  52;  dissolution 
of,  52;  edge  of,  [Fig.  45]  126;  level, 
123;  over  mountains,  [Fig.  51]  131; 
temperature  at  top  of.  153;  tempera- 
ture gradients  within  and  without, 
[Fig.  64]  173;  upper  portions  of,  185; 
violent  vertical  motions  in  middle  of, 
172 

Currents,  air:  ascending  and  descending, 
107;  convection,  extent  of,  51;  con- 
vectional,  retarded,  cause  of  cooling, 
264;  cold  lower,  in  ice  storm,  240; 


cross,  minor  vortices  in,  107;  devia- 
tions due  to  deflective  effect,  57;  in 
thunderstorm,  171;  oblique,  57; 
slow-moving,  of  lower  strata,  259, 
260;  stratification  and  interpene- 
tration  of,  78;  strength  of,  varies 
with  height,  107;  strongest,  location 
of,  107;  underrunning  cold,  in  thun- 
derstorm, 178;  upper  air,  123;  upper 
air  in  ice  storm,  240;  vertical,  effect 
of,  102:  see  also  Air;  Cyclones;  Winds 

Currents,  ocsan:  Antarctic,  73;  Arctic, 
73;  Argentine-counter,  72;  Austra- 
lian-counter, 72;  Bering,  73;  Cali- 
fornia, 74;  Cape  Horn,  72;  central 
counter,  73;  cold  Antarctic,  70;  cold 
Arctic,  70;  cold  ocean,  70;  Davidson, 
74;  equatorial,  74;  Georgia,  72,  73; 
Greenland,  73,  74;  Indian,  72; 
Japan,  73,  74;  Madagascar,  72; 
Northeast,  73,  74;  Pacific,  72;  Pacific 
Equatorial,  74;  Peru,  reports  on, 
issued  by  Hydrographic  Office,  72; 
South  Atlantic,  direction  of,  73; 
Spitsbergen,  73;  warm  ocean,  70 
Labrador:  73;  and  Bermuda  high,  69; 

in  June,  76 

southern  ocean:  direction  of,  72;  ve- 
locity of,  73 

Curvature,  gradient  and  rotation  gra- 
dient in  cyclone  and  anticyclones,  64 

Cycle  of  union  of  rain  drops  in  cloud,  171 

Cyclones,  77-86;  and  anticyclones  in 
sequence,  237;  and  ice  storm,  234; 
ascending  air  of,  54;  circulation, 
cause  of,  79;  cloud  distribution  in, 
[Fig.  42]  124;  cold-center,  79;  control 
of  weather  in  United  States,  69; 
counter-currents  underrunning  drift 
of,  78;  curvature  gradient  and  rota- 
tion gradient  in,  64;  definition  of,  77; 
direction  of  whirl,  78;  direction  of 
winds  in,  71;  earlier  theories  and 
observations,  77;  isobars  in  central 
region,  63;  motion,  as  indicated  on 
wind  charts,  220;  origin  of  name.  77; 
pressure  distribution  in,  [Fig  26]  81; 
pressure  in,  [Fig.  26]  81;  polar,  63; 
relation  of  cloud  motions  to,  110; 
southern,  dominated  by  northern 
anticyclones,  237;  temperature  dis- 
tribution in,  62,  [Fig.  25]  80;  tem- 
peratures of  front  and  back  of,  78; 
unit  for  rainfall  distribution,  220; 
vortices,  79;  warm-center,  79;  wind 
distribution  in,  [Fig.  24]  79;  wind 
velocity  in,  58;  winter,  and  snow, 
229:  see  also,  Air;  Currents;  Storms; 
Winds 

rotation:  determination  of  velocity,  62; 
elevation  and  increase  of  velocity  of, 
62;  level  of  maximum,  62;  of  air,  62 

Cyclostrophic  wind,  84;  formula,  84 

Dalton,  John,  cited,  209;  Dalton's  law,  21 


300 


THE  INDEX 


Dansey,  cited,  230 

Data  for  frost  work,  271 

Davidson  current,  74 

Davis,  W.  M.,  cited,  63,  77;  classification 
of  winds,  98;  cloud  measurements, 
110 

Davos  Observatory,  Swiss  Alps:  Dorno's 
records  of  spectra  of  solar  radiation, 
273 

Day:  cloudiness,  record  of,  for  two 
months,  [Fig.  56]  135;  mean  solar.  26, 
32;  sidereal,  32 

Dayton  (Ohio)  flood  of  1913;  Miami 
Street  Canal  Bridge,  [Fig.  97]  250; 
post-office  after  flood  of  1913, 
[Fig.  99]  252 

De  Blois,  L.  A.,  investigations  of  lightning 
discharge,  189,  190,  191 

Deflection:  and  pressure  distribution,  56; 
effect  of  air  currents,  deviations  due 
to,  57;  of  air  movement,  55,  56,  57 
force:  formula  for,  and  application,  56, 
57;  on  planetary  circulation,  56;  of 
unequal  absorption,  65 
— of  earth's  rotation,  56;  a  cause  of 
air  motion,  65;  Ferrel's  illustration 
of,  56 

De  Fonvielle,  W.,  cited,  11 

De  Jans,  C.,  cited,  191 

Delambre,  J.  B.  J.,  cited,  25 

Density:  and  position  of  cirrus  banks, 
118;  constancy  of,  law  of,  57;  de- 
creasing, and  increase  of  wind  veloc- 
ity, 81;  of  atmospheric  gases,  and 
formula  in  molecular  weight,  24; 
of  clouds,  figures  indicating,  118; 
symbol  for,  83;  unit  of,  26 

Department  of  Agriculture,  Bureau  of 
Soils:  Bulletin  68  on  soil  move- 
ment, 155 

Depression  center,  78 

Desert  geology,  155 

Deserts  and  monsoons,  217 

Detectors,  use  of,  for  lightning  records, 
190 

Deviating  force,  horizontal  component 
of,  in  high  altitudes,  59;  vertical 
component  of,  in  low  altitudes,  59 

Dew,  242,  243;  formation  of,  242,  243; 
international  symbol  for,  31;  meas- 
urements, 244;  records,  245;  deposit, 
244;  annual,  244;  measurements  of, 
245 

Dewar,  James,  cited,  22w,  245 

Diagram,  adiabatic:  [Fig.  57]  141;  appli- 
cation of  by  Neuhoff,  144;  methods 
of  making,  142;  methods  of  using, 
143 

Diameter,  semi-,  polar,  of  earth,  32 

Diameter,  sun's,  32 

Dines,  W.  H.,  annual  dew  deposit  in 
England,  244;  cited,  9,  37,  38n,  44, 
63,  80,  83,  107,  232w,  244;  light- 
weight meteorograph,  [Fig.  6]  18; 
tables  for  approximate  temperature 
gradient,  44,  45 


Discharge,  glow,  193 

Disease,  vitiated  air  and,  155 

Dissipators  to  prevent  formation  of  hail, 
242 

Distance:  horizontal,  symbol  for,  83; 
mean,  from  earth  to  moon;  from 
earth  to  sun,  32 

Disturbances,  atmospheric,  individual: 
frequency  and  path,  67;  relation  of, 
to  large  areas,  69 

Doldrums,  preceding  New  Orleans  hurri- 
cane, 258 

Dominion  Observatory,  Ottawa:  spectra 
of  lightning  by  Steadworthy,  194- 
197 

Dorno,  C.,  records  of  solar  radiation,  273 

Douglas,  A.  E.,  cited,  216 

Dove,  H.  W.,  cited,  77;  classification  of 
winds,  98 

Drainage  basins  of  Mississippi.  246; 
[Fig.  95]  247 

Drebbel,  C.  van,  cited,  2 

Drosometer,  Skinner's,  245 

Du  Bois,  estimate  of  salt  in  atmosphere, 
164 

Dune  control,  155 

Dust,  African,  in  southern  Europe,  156; 
and  haze,  160;  and  light  transmis- 
sion, 163;  and  microbes,  154-166; 
and  transparency  of  air,  160;  as 
nuclei  of  condensation,  156,  In; 
atmospheric,  and  sources  of,  155; 
content  of  atmosphere,  155,  7«; 
cosmical,  radio-active  in  upper  at- 
mosphere, 278;  counter,  157; 
counter,  Aitken's,  [Fig.  63]  158; 
estimate  at  Cincinnati,  156;  hot  air 
of  cities  as  carriers,  160;  in  air,  role 
of,  in  absorbing  and  radiating  heat, 
7n;  in  unsaturated  air,  160;  obscura- 
tion by,  157;  with  affinity  for  water, 
161;  without  affinity  for  water,  162 
particles:  condensing  power  of,  160; 
determine  color,  161;  distinct  from 
gaseous  particles,  162;  in  air,  number 
of,  [Aitken's  table]  160;  in  fogs,  161; 
vapor  on,  160 

volcanic:  155;  color  effect  of,  163;  from 
Krakatau  eruption,  101;  from  erup- 
tions and  transparency  of  air,  In 

Dynamic  heights,  Bjerknes'  classification 
of  adiabatic  gradients  according  to, 
139 

Dynamical  heating  and  cooling  of  air,  41 

Dynamical  units,  34 

Dyne:  26;  weight  of,  28 

Earth:  angular  velocity  of:  see  Earth, 
rotation;  mass  of,  32;  mean  density 
of,  32;  mean  temperature,  288;  nega- 
tively charged,  199;  radius  of,  sym- 
bol for,  83;  radioactive  content  of 
atmosphere  over,  201;  speed  of,  276; 
to  moon,  mean  distance  from,  32; 
to  sun,  mean  distance  from,  32; 


THE  INDEX 


301 


and    sun:,  relative    position    of,    273; 
relative    sizes    of,     [Fig.     112]    274; 
varying  distance  between,   276 
station:    deflecting    force    of,    56;    de- 
flecting   effect    on    air    velocity,    55; 
effect  of,  on  air  movements,  54 
— angular  velocity  of,  56,  57;  formula, 
57;  of  point  on  earth's  surface,   62; 
symbols,  84 

surface:  and  atmosphere,  relative 
movements  of,  60;  and  ocean,  rela- 
tive movements  of,  60;  area  of,  32; 
motions  of,  and  relative  motions  of 
atmosphere,  60 

East  Gulf  storm,  87 

Easterlies,  upper,  100 

Eastman,  cited,  154« 

Eclipse:  record,  lunar,  132;  moonlight 
record  of,  [Fig.  53]  133;  total  solar, 
and  cloud  record,  136 

Eddy,  W.  A.,  cited,  8 

Edge  of  a  cumulus  cloud,  [Fig.  45]    126 

Egnell's  law:  57,  81;  conditions  that 
verify,  85,  86 

Ekholm,  Dr.  N.:  cited,  58,  63,  90;  cloud 
measurements,  110;  summary  of 
deviations  due  to  deflective  'effect 
57;  terms  for  pressure  change  areas, 
90 

Elasticity,  equation  of,  39,  82 

Electric  arc,  temperature  of,  288 

Electric  potential,  observations  of,  by 
McAdie,  8 

Electricity:  available,  determining  factor 
of  types  of  lightning,  193;  charge 
carried  by  rain,  169;  effects  of  thun- 
derstorm, simulative  experiments 
for,  170;  loss  of,  by  earth,  200; 
origin  of,  in  thunderstorms,  171,  172; 
proportion  of  positive  and  negative 
in  snow,  170;  resistance  of  tempera- 
ture for  disappearance  of,  287; 
separation  of  drops  in  cloud  by,  171; 
spark  discharge,  growth  of,  [Fig.  69] 
186;  atmospheric:  167-204;  as  inci- 
dent to  storm,  168;  normal,  199; 
origin  of,  168 
current:  densities  in  rain,  169;  vertical 

in  snowfall  and  rainfall,  170 
discharge:  an  explosion  wave,  187;  and 
condensation,  148;  and  precipitation, 
148;  heavy  rainfall  with,  207 
negative:    frequency    of,   in    periods    of 
rainfall,    170;   frequency   not   in  any 
particular  period  of  storm,    170;   in 
earth,  199 

positive:  excess  of,  in  rain,  169;  and 
negative,  in  rain,  169;  and  negative, 
periods  of,  in  rain,  169;  and  negative, 
quantities  of,  in  rain,  169 

Electrification:  and  air  friction,  170;  and 
freezing  and  thawing,  170;  of  snow, 
170 

Electrolytic  decomposition  of  water  vapor 
by  lightning,  189 


Electromometer,  Benndorf,  used  in  Simp- 
son's observations,  169 

Electrons,  negative  in  cumulus,  172 

Elevation:  and  increase  of  velocity  of 
cyclonic  rotation,  62;  of  maximum 
rainfall,  212 

Elias,  Hermann,  cited,  9 

Emissivity  of  heat,  38 

Energy  used  in  expansion,  43;  kinetic,  26; 
determining  temperature,  33;  po- 
tential, 26 

Emden,  Robert,  cited,  50 

Epsom  Station,  Surrey,  cloud  record  by 
S.  C.  Russell,  134 

Equal  areas,  law  of,  57 

Equal  mass  transport,  law  of,  at  all 
heights,  81 

Equation:  characteristic,  for  air,  40;  for 
change  of  pressure  gradient  with 
height,  85;  of  elasticity,  39,  82;  of 
relation  between  velocity  of  wind 
and  pressure  gradient,  84;  "equation 
of  state,"  6:  see  Boyle-Gay-Lussac 
law;  Shaw's,  for  strophic  balance,  84 

Equator:  circumference  at,  32;  currents, 
74;  regions  at,  rainfall  in,  217 

Equilibrium:  adiabatic,  137;  stable,  con- 
ditions of,  for  saturated  air,  138 

"Equipluves,"  218 

Equivalence  of  pressure  distribution  and 
wind,  law  of,  84 

Erg,  26,  30,  42 

Erk,  Fritz,  cited,  1 1 

Espy,  James  P.,  cited,  77 

European  System,  Old,  of  pressure 
units,  [Table]  30 

Evaporation:  152;  affected  by  foreign 
matter,  154;  cause  of  dust  in  air,  155; 
during  rainfall  as  cooling  agent  of  a 
thunderstorm,  176,  177;  effect  on 
cloud  motion,  151;  increase,  153; 
temperature  of,  recording  of,  by 
thermograph,  268;  [Fig.  Ill]  269;  of 
water,  total  heat  required  for,  270 
rate  of:  and  saturation  deficit,  241;  and 
wind  movement,  241;  changes,  ob- 
served by  Robertson,  10;  formula 
for,  241 

Expansion:  and  cooling,  137;  effect  on 
cloud  motion,  151;  energy  used  in,  43 

Fahrenheit  scale,  35:  see  also  Scales 

Fair  weather  and  cirro-cumulus,  130 

Falls,  90 

"False  cirrus,"  117 

Farrar,  Prof.  John,  cited,  77 

Fassig,  Prof.  O.  L.,  cited,  65,  211,  66w 

Feet  into  meters,  conversion  of,  [Table  2] 

279 

Fenyi,  cited,  50 

Ferdinand  II,  of  Tuscany,  cited,  5 
Fergusson,    S.   P.,   cited,    9,    211;    cloud 
measurements  by,  110,  133;  modifi- 
cation of  the  Pole-star  recorder,  133; 
rain  gauge,  210 


302 


THE   INDEX 


Ferrel,  William:  cited,  8,  55,  56,  60,  77, 
22n,  268;  formula  for  determining 
pressure  of  aqueous  vapor,  268; 
illustration  of  deflecting  force  of 
earth's  rotation,  56;  scheme  of 
planetary  circulation,  56 

Fertilizing  agents  from  lightning  dis- 
charge, 199 

Finley,  J.  P.,  cited,  93 

Fitzroy,  R.,  cited,  112 

Flattening,  reciprocal  of,  at  poles,  32 

Flocciforms,  [Clayton's  Chart]  119:  see 
also  Clouds 

Floods  and  notable  storms,  246-258; 
and  storm  frequency,  248;  forecast- 
ing of,  246;  snow  surveys  for  predic- 
tion of,  225;  relation  with  duration 
of  winds,  246;  storm  frequency  and, 
246 

warnings:  accuracy  of,  246;  by  Professor 
Stearns,    weather   observer,    at    Sea- 
brook,     Texas,     256;     by     weather 
Bureau  at  Galveston,  256 
of    1912:     in     Mississippi     basin,     248; 
Pacific    storm,     248;    North    Pacific 
storm,  248;  West  Gulf  storm,  248 
— precipitation  in,  [Fig.  98]  251;  dur- 
ing flood  in   Mississippi  basin,   [Fig. 
96]  249 

of  1913:  and  movement  of  hyperbars, 
250;  in  Mississippi  basin,  248;  in 
Ohio  Valley,  most  disastrous,  250; 
low  pressure  lane  between  two  rain 
areas,  250;  temperature  during,  252 
— Dayton  (Ohio) ,  Miami  Street  Canal 
Bridge,  [Fig.  97]  250;  post-office 
after,  [Fig.  99]  252 

Foehn:  adiabat,  diagram  of,  [Fig.  59]  145; 

cloud  bank,  151;  origin  of  word,  103; 

wind:  71,  103;  diagram  of,  [Fig.  58]  145; 

place  of  origin,  103;  precipitation  in, 

145 

Fog,  amount  of  water  in,  150;  and  clouds, 
originating  of,  150;  dissipation, 
temperature  preceding,  152;  forma- 
tion and  dissipation,  Coast  Guard 
records  of,  148w;  international  sym- 
bol for,  31;  low,  [Fig.  36]  114; 
morning,  rising,  [Fig.  35]  113;  per- 
sisting, 162;  shallow,  from  tem- 
perature inversion,  150;  type  of 
clouds,  112 
dust  particles:  161;  diameter  of,  150; 

sunshine  nuclei,  162 
sea:  150;  augmented  by  radiation  fog, 
[Fig.  35]  113;  formation  of,  148;  off 
Newfoundland,  temperature  and 
path  of  air  during,  [Fig.  61]  149; 
temperature,  150 

Foot-pounds,  26,  27 

Force:  26;  measure  of,  27;  of  atmospheric 
pressure,  units  of,  27;  unit  of,  26; 
deflecting:  application  of  formula  for, 
57;  of  earth's  rotation,  56;  Ferrell's 
illustration  of,  56;  on  air  move- 
ment, 56 


Forecast :  of  temperature  from  velocity  or 
direction  of  air  flow,  264;  of  water 
supply  from  snow,  226 

Forecasting:  by  means  of  wireless  re- 
ceivers, 189;  clouds  in,  125;  floods, 
246;  heat  divergence  obtained  for, 
140;  storms,  87-96;  weather  changes, 
clouds  unreliable  in,  125 

Forest  Service:  data  of  trees  struck  by 
lightning,  203 

Forster,  T.  I.  M.,  cited,  112 

Foucault  pendulum  experiment,  62 

Fowle,  F.  E.,  Jr.:  cited,  In,  164,  277 

Fox,  Philip,  spectrum  of  lightning,  [Fig. 
75]  194 

"Fracto-cumulus,"  115 

"Fracto-nimbus,"  115 

"Fracto-stratus,"  117 

Frankenfield,  H.  C.,  cited,  246,  254 

Franklin's  kite  experiments,  8,  167; 
observations,  74,  77 

Free,  Edward  E.,  cited,  155 

"Freezes,"  259 

Freezing  and  electrification,  170;  tem- 
peratures at  high  levels,  185 

Friction:  in  a  horizontal  parallel  air  cur- 
rent, 58;  effect  of:  on  deviation  of  air 
motion,  58;  on  wind  velocity,  58 

Fritsche,  H.,  cited,  112 

Frosts,  259-272;  and  inversions,  [Fig. 
107]  165;  and  stagnant  air,  260;  and 
vertical  movements  of  air,  259; 
deposit  governed  by  mixing  ratio, 
267;  heavy,  and  region  of  lowest 
temperature,  270;  hoar,  interna- 
tional symbol  for,  31;  local  condi- 
tions favoring,  264;  types  of  storm 
movement  preceding,  264;  typical 
late  spring,  [Fig.  107]  265 
conditions:  close  proximity  of  unlike 
strata  in,  268;  in  billows  and  bars, 
268 
forecast:  by  flow  of  surface  air,  259; 

from  gusty  winds,  264 
formation:    242,    259;    and    strong    air 
streams,    259;    conflicting    processes 
in,     262;    crystals,    [Fig.     108]    266, 
[Fig.  109]  267;  various  processes  of, 
260;  water  vapor  in,  262 
work:  conversion  table  for,    270;   data 
for,   271 

Frostwork,  international  symbol  for,   31 

Frozen  precipitation:  forms  of,  230;  rain- 
drops, sizes  of,  232 

Fry,  F.  R.,  and  work  of  Smoke  Abate- 
ment League  of  Cincinnati,  156 

Fusion,  latent  heat  of,  39 

Gal:  28;  unit  of  acceleration  of,  26,  28 
Gale,  international  symbol  for,   31 
Galileo,  cited,  2,  5 

Galveston:   flood,   cause   of,    253;   hurri- 
canes,  paths  of,   in    1900  and    1915, 
[Fig.  101]  254 
storms:    252;    comparative    intensities, 


THE  INDEX 


303 


255;  interval  between,  253;  paths  of, 
254;  wind  velocities,   255 
- — of  1900,  252 ;lowest  recorded  baro- 
metric pressure,  252 
— of  1915,  course  of,  253;  warnings  of 
Weather  Bureau  preceding,  256 
Gaseous  and  dust  particles,  162 
Gases,  convection  of,  23;  densities  of,  24; 
distribution  of,  in  atmosphere,  [Fig. 
8],  23;  noble,  7;  volume  percentages, 
22 

atmospheric:  distribution  of,  21,  22; 
formula  for  densities  of  in  molecular 
weight,  24 

Gas  constant:  for  air  not  constant,  21; 
Shaw's  formula  for,  40;  value  of  in 
characteristic  equation  for  air  [Table] 
40 

Gaster,  cited,  112 
Gauss,  K.  F.,  cited,  25 
Gay-Lussac,  J.  L.,  experiments  on  atmos- 
pheric magnetism,   10;  Gay-Lussac's 
law,  39 

Gellibrand,  Henry,  cited,  54w 
Geo-coronium,  7 
Geodetic  data,  general,  31 
Geography,  local,  and  snow  distribution, 

224 

Georgia  current,  72,  73 
Geostrophic  wind:  84;  formula,  84 
Givre:  233,  242;  formation  of,  243;  from 

condensation,  244;  of  deposit,  244 
"Glacier  burn,"  274 
Glaciers     and     marginal     ice     fields     in 

Antarctica,  71 
Glaisher,  James,  cited,  10 
Glatteis,  231,  244 
Glaze,  244 

Glazed  frost,  230,  244 
"Glory,"  166 
Gockel,  Albert,  cited,  274 
Goethe,  J.  W.,  interest  in  meteorological 

phenomena,  112w 

Gold,    E.:   cited,    46,    49,    53,    63,    232w; 
diagram  of  annual  variation  in  height 
of  stratosphere,  [Fig.  12]  52;  table  of 
mean  height  and  actual  temperature 
of  stratosphere,  50 
Gold,  melting  point  of,  288 
Grade,  27 

Gradient:  acceleration  of  air  motion 
produced  by,  58;  and  wind  velocity, 
relation  between,  58;  barometric,  by 
velocity,  balance  of,  63;  negative, 
139;  velocities,  necessity  of  calcu- 
lating 63 
curvature  and  rotation:  of  cyclones, 

63,  64;  of  anticyclones,  63,  64 
potential:  and  thunderstorm,  199; 
measurements  by  Swann,  201;  ob- 
servations of,  201;  of  charge  in 
different  periods  of  rainfall,  170; 
recorded  on  Benndorf  electromome- 
ters  at  Simla,  169;  steepness  of,  and 
types  of  discharge,  193 


pressure:  and  wind  velocity,  58;  equa- 
tion  for   change   of,    85;   horizontal, 
symbol    of,     83;    maintenance,    and 
underlying  thermal  structure,  81,  82 
rotation  and  curvature:  of  cyclones,  63, 

.64;  of  anticyclones,  63,  64 
temperature:  actual,  44;  and  cloud 
formation,  53;  approximate,  [Tables] 
44,  45;  cessation,  level  of,  46;  control 
cloud  movements,  127;  decrease  of 
sign  in,  not  caused  by  cloud  forma- 
tion, 53;  effect  of  season  and  latitude 
on,  46;  equilibrium,  stable,  condition 
for,  138;  negative,  condition  for,  139; 
per  thousand  meters,  45;  reversion, 
level  of,  46;  within  and  without 
cumulus  clouds,  [Fig.  64]  173 
— adiabatic,  at  different  heights, 
[Table]  38;  formula  for,  37;  of  satu- 
rated air,  [Table]  138;  variation  of, 
138 

— horizontal,    and    storm    intensity, 
89;  symbol  for,  83 

wind:  computed  from  pressure  dis- 
tribution, 106;  formula  for  velocity, 
106 

Gram,  25 
Gram-calorie,  34 

Grand  Bank,  study  of  ice  conditions  at,  75 
"Grand  centers  of  action"  of  high  and 

low  pressure  areas,  65 
Graupel,  230,  231 
Gravitation,  28n 

Gravity:  28,  28w;  absolute,  57;  apparent, 
57;  cause  of  air  motion,  65;  compo- 
nents of,  effect  of,   56;  standard,  42 
normal     acceleration     of:     [Table]     28; 

symbol  for,  83 

Great'  Blizzard  of  New  York,  89w 
Great  Lakes,  storms  over,  and  highs,  88 
Greenland  current,  73,  74 
Grimaldi,  F.  M.,  cited,  21;  cloud  measure- 
ments of,   110 

Gross,  Lieutenant  H,,  cited,  153 
Guldberg,  cited,  63 
Gulf  Stream,  72,   73,   74,   76;  as  climate 

control,  74 

Gyration,  center  of,  78 
Gyroscopic  motion:  59;  absolute  point  of 
view,  60;  experiments  from  absolute 
point  of  view,  60;  experiments  from 
a  relative  point  of  view,  59;  hydro- 
dynamical  experiments,  59;  impres- 
sions of,  59;  relative  point  of  view, 
56,  60 

Hadley,  John,  theory  of  trades,  55 
Hagstrom,  J.  O.,  cloud  measurements  of, 

110 

Hail:  140,  171,  230,  231;  and  roll  scud, 
186;  dissipators  of,  242;  formation 
at  high  levels,  186;  in  thunderstorms, 
184;  international  symbol  for,  31; 
location  in  storm,  186;  stage,  tem- 
perature of  air  at,  142 


304 


THE   INDEX 


Hailstone,  formation  of,  185;  nucleus  of, 
185;  size  of,  and  convection  current, 
185 

Hales,  Wm.,  cited,  6 

Halley,  Edmund,  magnetic  survey  of 
ocean,  54,  55 

Halos:  164;  and  cirro-stratus,  164;  and 
precipitation,  164;  colors  of,  164; 
data  regarding,  165;  effects,  163; 
lunar,  international  symbol  for,  31; 
of  Bravais,  165;  solar  international 
symbol  for,  31 

Hamburg  Observatory,  undirectional  dis- 
charge of  lightning  proved  by  Dr. 
Walters.  186 

Hann,  J.,  cited,  22n,  66,  77,  79,  146; 
table  of  adiabatic  gradients,  138; 
table  of  condensation,  140 

Hargrave,  Lawrence,  inventor  of  box 
kite,  8 

Harmattan  type  of  wind,  104 

Hastings,  Charles  S.,  cited,  164,  165 

Haze:  160;  and  dust,  160;  international 
symbol  for,  31 

Heaping  of  ocean  water,  60;  see  Horse 
latitudes 

Heat,  a  form  of  energy,  33;  and  ultra- 
violet radiation,  274;  distribution 
and  circulation,  44;  distribution  and 
radiation,  44;  divergence  obtained  for 
forecasting,  140;  equivalent  of  work; 
see  Mechanical  Equivalent  of  Heat: 
kinetic  energy  transformed  into,  41; 
measurement  of,  34;  nature  of,  33; 
potential  energy  transformed  into, 
41;  role  of  dust  in  air  in  absorbing 
and  radiating,  In;  specific,  34;  stag- 
nation, 155;  total,  required  to 
evaporate  water,  270;  units  of, 
large  and  small,  42;  unequal  absorp- 
tion of,  as  deflecting  force,  65 
Joule' ^  equivalent:  38;  conversion  factor 

for,  38 

latent:    38,     152;     consideration    of,    in 

problems  of  heating  water,   270;  of 

aqueous    vapor,    formula    for,     271; 

of  fusion,  39;  of  vaporization,  39 

loss:  adiabatic  rate  in,  44;  and  gain  of, 

137;   under  diminished  pressure,  43 

mechanical    equivalent    of:     38;    under 

standard  gravity,  42 

Heating:  and  cooling,  dynamical,  of  air, 
41;  unequal,  of  equatorial  and  polar 
regions,  65 

Height  and  temperatures,  mean,  at  base 
of  stratosphere,  [Table]  50;  atmos- 
pheric: see  Aurora;  of  atmospheric 
pressure,  27 

Hellman,  G.,  cited,  1,  2,  156;  classifica- 
tion of  snow  crystals,  221;  frozen 
precipitation,  230;  observations  of 
snow  crystals,  221;  vapor  condensa- 
tion, 2  3  On 

"Helm  and  Bar,"  212 
Helmert,  F.,  formula  of,  28n 


Helmholtz,  H.  von,  observations  of  cloud 
billows,  123,  125 

Helmholtz,  Robert  von,  cited,  148,    151 

Helm  wind,  151 

Helvelius,  cited,  165 

Helium:  in  atmosphere,  7;  boiling  point 
of,  287;  liquefied,  temperature  for 
maximum  density  of,  287;  percen- 
tage in  air,  22;  solid,  temperature 
for  obtaining,  287 

Henry,  Alfred  J.,  cited,  89,  90,  202 

Hepworth,  Commander,  R.  N.  R.,  cited, 
75 

Herbertson,  A.  J.,  cited,  217 

Hergesell,  H.,  cited,  9,  11,  15 

Hermite,  Charles,  sounding  balloons,   1 1 

Hero  of  Alexandria,  cited,  2 

Hertz,  Heinrich:  classification  of  con- 
densation, 140;  diagram,  142;  Neu- 
hoff's  modification  of  diagram,  268 

Hewlett,  cited,  201 

Highs:  continental,  in  Idaho,  67;  hurri- 
canes directed  by,  88;  influence  of, 
on  storms  over  Great  Lakes,  88; 
movements  of,  and  pressure  change 
areas,  90;  northern,  238;  paths  of,  in 
United  States,  [Fig.  28]  91;  true  arc, 
temperature  of,  288 

Hildebrandsson,  H.,  cited,  110,  112,  113, 
118,  121 

Hill,  cited,  154 

Hoarfrost:  230,  242;  formation  of,  243; 
international  symbol  for,  31 

Hoffmeister,  cited,  20 

Hoffmeyer,  cited,  65 

"Holes  in  the  air,"  107 

Horsburgh,  James,  cited,  77 

Horse  latitudes:  60;  heaping  of  ocean 
water  at,  60;  origin  of  term,  60w 

Horse-power,  26;  standard  value  of,  27 

Horton,  Albert  H.,  cited,  248w 

"Hot  winds,"  104 

Hours  for  recording  state  of  sky,   133 

Howard,  Luke:  classification  of  clouds, 
111,  112,  118,  244;  rain  gauge,  245 

Humidity:  above  fog  layer,   153;  change 
in,  noted  from  balloon,   10;  difficult 
to  record,  151,  265;  fall  of,  with  rise 
of  temperature,  265;  maps  of  change 
in,  89;  specific,  267 
absolute:  and  vapor  pressure,  difference 
between,    267;    on   day   of   thunder- 
storm, 183;  relations  of,  201 
relative:    in    rainfall    and    in    thunder- 
storm,   177;    misleading    term,    265; 
on  day  of  thunderstorm,  183 
Humphreys,  W.  J.,  cited,  50,  65,  66w,  69, 

168,  191;  quoted,  21,  198 
theory:  of  hyperbars,  66;  of  turbulence 
in  thunderstorms,  173;  on  cooling  of 
thunderstorm,   176 

Huntington,  Ellsworth,  cited,  154w,  216 
Hurricanes:  77;  course  directed  by  highs, 
88;  influence  of  rainfall  on,  88;  inter- 
val between,   on   Texas  coast,    253; 


THE  INDEX 


305 


of  August,  1915,  barometric  pressure 
at    Houston,    [Fig.    100]    253;    three 
successive,  on  Gulf,  257;  tropical,  84 
of  1900  and  August,  1915:  comparison 
by  Frankenfield,  254,  255;  paths  of, 
[Fig.   101]  254;  wind  velocities,  255 
West  Indian:   88;   of   1915,  rainfall  in, 
[Fig.  104]  257 

Hutton,  James,  discussion  of  mixing  ratio, 
146 

Hydrogen:  boiling  point,  287;  discovery, 
6;  in  air,  6w2;  gas  thermometer,  36; 
molecular  weight,  24;  percentage  in 
air,  22;  peroxide  from  lightning  dis- 
charge, 198 

Hydrographic  Office  of  the  United  States 
Navy:  Gulf  Stream  work  by  Lieut. 
Soley,  74;  pilot  charts  of  trades 
issued  by,  98;  reports  on  flow  of 
ocean  currents,  72 

Hydrography,  70 

Hydroplanes,  use  of,  in  ocean  surveys,  75 

Hygrograph,  266 

Hyperbars:    65;    Azores,    66;    Bermuda, 
effect  on  Atlantic  coast,    69;  conti- 
nental, 66;  North  America,  66;  over 
Indian  Ocean,    66;   persistency,  and 
character    of     season,    66;     seasonal 
reversals,    66;   Southern  Pacific,  65; 
winter,  position  of,  66 
location:  65;  on  Pacific,  65;  on  Atlan- 
tic, 65 
movements:  98;  and  flood  of  1913,  250; 

Ice:  coating,  international  symbol  for,  31; 
conditions  in  Labrador  current,  75; 
drifting,  direction  of  South  Atlantic 
currents,  shown  by,  73;  flow,  con- 
stant, affecting  ocean  currents,  71; 
rain,  230,  231;  cause  of,  230;  water, 
British  thermal  units  required  to 
change  to,  271 

crystals  and  halos,  164;  floating,  inter- 
national symbol  for,  31 
storms:  231,  [Fig.  91]  240;  [Fig.  93] 
242;  [Fig.  94]  243;  and  cyclones 
and  anticyclones,  234;  Blue  Hill, 
[Fig.  92]  241;  chart  of  conditions 
during,  [Fig.  88]  234;  classification 
of,  237;  combinations  and  conditions 
of,  238;  distribution  of,  by  months, 
238;  northeasterly  type  of,  234,  237; 
northwesterly  type  of,  236,  237; 
southerly  type  of,  234;  sudden 
changes  in  wind  and  temperature 
of,  238;  temperature  record  during, 
[Fig.  89]  235;  thermograph  curves 
showing  changes  in,  [Fig.  90]  239; 
two  types  over  same  area,  236;  upper- 
air  conditions  in,  233;  various  types 
of,  230;  wind  conditions  which  pro- 
duce, 234 

Icebergs,  75 

Icelandic  infrabar,  66 

Inches  into  millimeters,  conversion  of, 
[Table  1]  279 

21 


India,  northern,  rainfall  in,  213 

Indian  current,  72 

Indian  Meteorological  Department,  Simla: 

Dr.  C.  G.  Simpson's  observations  on 
atmospheric  electricity,  168-172 

Indian  Ocean  hyperbar,  66 

Infrabars:  65;  Aleutian,  66,  67;  Icelandic, 
66;  location,  65;  movements  of,  98 

Insolation,  unit  of,  and  variations  of  dura- 
tion of  sunshine,  [Table]  276 

Instrument  for  measuring  cloud  heights, 
[Fig.  43]  125 

Intel-conversion  of  nautical  and  statute 
miles,  [Table  5]  281 

"Interfaces,"  wave  reflection  of  thunder 
from,  188 

International  Commission  for  Scientific 
Aeronautics:  authority  in  upper-air 
investigations,,  13;  congresses  held, 
13;  soundings  at  Uccle,  [Table]  47 

International  Committee  for  Scientific 
Aeronautics,  atlas  of  cloud  forms 
prepared  by,  113;  barometric  tend- 
ency defined  by,  89;  Beaufort  wind 
scale,  equivalent  values  of,  290; 
standardization  of  thermometers  for 
balloon  explorations,  12 

International  Committee  on  Weights  and 
Measures:  standard  of  normal  accel- 
eration adopted  by,  28n 

International  Meteorological  Conference 
at  Paris:  committee  appointed  to 
organize  international  balloon  as- 
cents, 11;  kites  recommended  for 
upper  air  investigations,  8 

International  Meteorological  Congress 
at  Munich:  committee  appointed  for 
preparation  of  atlas  of  cloud  forms, 
113;  Aerological  Congress  at  Monaco : 
Koppen's  proposal  for  new  units  of 
pressure,  29;  Aerological  Congress  at 
Vienna  (1873):  adoption  of  inter- 
national symbols,  31 

Intervals  of  thunder,  189 

Invar,  melting  point  of,  288 

Inversion:  139;  and  frosts,  [Fig.  107]  165; 
major  and  minor,  46;  negative  gra- 
dient, 139;  of  temperature,  records 
of,  262;  temperature  on  sea  fog,  150; 
unusual  type,  [Fig.  106]  263;  upper, 
46;  winter  type,  [Fig.  105]  261 

lonization:  amount  of,  determining  type 
of  lightning,  193;  showing  radiation 
in  upper  atmosphere,  278 

Ions,  positive  and  negative,  in  broken 
drops,  170 

"Irisation,"  166 

Iron:  cast,  melting  point  of,  288;  melting 
point  of,  288 

Irregularity  of  waves  in  thunder,  187 

Isallobars  and  winds,  relations  between, 
90 

Isobars:  29,  82;  closeness  of,  in  central 
region  of  cyclone,  63;  separation  of, 
in  inner  region  of  an  anticyclone,  63 


306 


THE  INDEX 


Isohyets,  217 

Isomers,  217 

Isothermal  change  of  altitude,  268 

Isothermal  layer,  46 

Isothermal  stratum,  49,  [Fig.  11]  51 

Isotherms:   75;  Antarctic,   70;  trend  of, 

and  laws,  88 
Isothermic    atmosphere,    conditions    for, 

139 

Jackson,  A.  H.,  cited,  248w 

Japan  current,  73,  74 

Jeffries,  Dr.  John,  balloon  expedition  of,  9 

Jericho,  Vermont,  Bently's  study  of  snow 

crystals  at,  221 
Jesse,  E.,  cited,  112 
Jevons,  W.  S.,  cited,  112 
Johnston,  Captain  C.  E.,  cited,  76 
Johnston,  H.  F.,  cited,  201 
Jost,  E.  H.  R.,  cited,  17 
Joule,  J.  P.:  cited,  38;  Joule's  equivalent, 

38;  conversion  factor  for  equivalent, 

38 

Kaehler,  Karl,  cited,  201 

Kaemtz,  L.  F.,  cited,  112 

Kassner,  Carl,  cited,  112 

Katallobar,  90 

Katmai,  eruption  of,  and  atmospheric 
dust,  163 

Kelvin,  Lord,  quoted,  202;  absolute 
temperature  system,  33 

Kew  Observatory,  cloud  measurements 
by  photography,  110;  four  ascents 
in  1852  for  aerial  explorations,  10 

Khamsin  (southeast  wind),  104 

Kidson,  E.,  cited,  201 

Kiessling,  Carl  J.,  cited,  148 

Kilobars:  30;  conversion  of  millimeters 
to,  [Table  8]  284 

Kilogram:  25;  -meter,  42;  prototype,  25 

Kilometers,  conversion  of  miles  into, 
[Tables  3,  4]  280 

Kilowatt,  27 

Kimball,  H.  H.,  cited,  In,  20w,   164,  277 

Kinetic  energy:  26;  compression  and,  41; 
determining  temperature,  33;  trans- 
formed into  heat,  41 

Kites :  7,  16;  Archibald  first  to  photograph 
from,  8;  box,  8;  experiment  at  Blue 
Hill,  167;  first  use  of,  for  air  investi- 
gations, 8;  first  use  of,  at  sea,  9; 
flying  in  icestorm,  236;  Franklin's 
experiment,  167;  greatest  altitude, 
reached  by,  9;  meteorographs,  9; 
observations  with,  13 

Klein,  J.  T.,  cited,  112 

Klotz,  Otto,  cited,  28n 

Koniscope,  Aitken's,  for  determining 
size  and  number  of  dust  particles,  161 

Kb'ppen,  W.,  new  pressure  base,  29; 
charts  of  wind  movement,  70,  [Fig. 
22]  71;  [Fig.  23]  72;  97 

Korean  Meteorological  Observatory: 
early  rainfall  observations,  209;  his- 


torical data  of  rain  gauges,   by   Dr. 

Y.  Wada,  207 
Krakatau,  eruption  of,  and  atmospheric 

dust,  101,  163 
Kramer,  A.,  cited,   18 
Krebs,  Wilhelm,  cited,  213 
Krypton  in  atmosphere,  7 
Kuro  Shio:  see  Japan  current 

Labrador  current:  73;  climatic  effect  of, 
74;  effect  of,  on  Bermuda  hyperbars, 
69;  ice  conditions  in,  75;  in  June,  76 

Lake  region,  storms  over,  90 

Lamarck,  J.  B.  P.  A.,  classification  of 
clouds,  110,  111 

Land  breeze,  98:  see  also  Breezes 

Langley,  S.  P.,  cited,   136 

Laplace,  P.  S.,  cited,  37;  Laplace's  for- 
mula for  velocity  of  sound,  37 

La  Soufriere,  eruption  of,  and  atmos- 
pheric dust,  163 

Latent  heat:  38,  152;  consideration  of,  in 
problems  of  heating  water,  270; 
of  aqueous  vapor,  formula  for,  271; 
of  condensation  at  high  level,  185; 
of  fusion,  39;  of  vaporization,  39 

Latitude,  symbol  for,  84 

Latitudes,  horse,  60;  see  also  Horse  lati- 
tudes 

Lava,  boiling,  temperature  of,  288 

Lavoisier,  A.  L.,  cited,  6 

Law  of  Boyle  and  Mariotte,  39 

Law  of  Charles  and  Gay-Lussac,  39 

Law  of  Dalton,  21 

Lead,  melting  point  of,   288 

Le  Conte,  Prof.  J.  N.,  snowfall  records, 
226 

Lee,  Professor  F.  S.,  cited,  154w 

Lenard,  P.,  cited,  205 

Length,,  unit  of,  25 

"Leo,"  28n 

Leslie,  Sir  John,  cited,  146 

Leste  type  of  winds  of  Madeira  Islands, 
104 

Letters,  international,  of  cloud  names, 
113,  115,  117 

Leveche  type  of  winds  of  Spain,  104 

Ley,  W.  Clement,  investigations  of  cloud 
types,  110;  classification  of  clouds, 
112 

Light:  from  clouds,  analysis  of,  by  Welsh, 
10;  pillar,  165;  time  to  traverse  mean 
radius  of  earth's  orbit,  32;  transmis- 
sion and  dust,  163;  velocity  of,  32 

Lightning,  186;  and  raindrops,  [Table] 
207;  ball,  192,  193;  beaded,  193; 
De  Blois'  investigations  and  records 
of  character  of  discharge,  189,  190; 
brush  discharge  of,  192,  193;  chemi- 
cal effects  of,  198;  coronal  discharge 
from  storm  cloud,  193;  distant,  inter- 
national symbol  for,  31;  electrolytic 
and  thermal  decomposition  of  water 
vapor  by,  189;  explosive  effects  of, 
198;  flash  and  dark  flashes,  [Fig.  73] 


THE   INDEX 


307 


191,  193;  glow  discharge,  193;  heat, 
193;  intensity  and  excessive  rain 
formation,  189;  ionization  determin- 
ing kind  of,  193;  origin  of,  from 
cumulus,  171,  172;  peaks  of,  cause  of, 
191;  photograph  of,  taken  in  day- 
light, [Fig.  74]  192;  pressure  waves 
and  sound  from,  189;  protection 
from,  and  efficiency  of  rods,  202; 
quantity  of  electricity  determining 
kind,  193;  return  strokes,  193; 
rocket,  192,  193;  sheet,  193;  single- 
and  multiple-peak  discharges  of,  190, 
191;  steeple-front  discharge,  190; 
steepness  of  potential  gradients  in 
determining  kind  of,  193;  thunder 
and  raindrops,  velocity  of,  184;  undi- 
rectional,  186 

discharge:   character   of,    189;    through 
clouds,  [Fig.  72]  190;  violent  pressure 
fluctuations  in,  188 
energy:  amount  of ,  189;  transformation 

of,  189 

spectra  of:  [Fig.  75]  194;  [Fig.  76]  195; 
[Fig.  77]  196;  lines  in,  [Table]  196, 
197;  streak  and  sheet  lightning, 
difference  between,  193 
streak:  (sequent  discharge),  rotating 
camera,  [Fig.  71]  187;  stationary 
camera,  [Fig.  70]  187 
stroke:  destructive  effects  of,  198; 
liability  of  trees  to,  203;  percentages 
of  trees  struck  by,  [Table]  203;  posi- 
tive (from  earth  to  clouds),  190; 
positive  (from  clouds  to  earth),  190; 
resuscitation  from  204. 

Lindenberg  Observatory,  balloon  expedi- 
tion and  ascensions  on  the  Victoria 
Nyanza,  15,  49;  most  complete  for 
air  research,  15 

Lines  in  spectrum  of  lightning,  [Table] 
196,  197 

Linnaeus'  scale:  see  Scale,  Centigrade 

Liquid  air,  temperature  for,  287 

Liquid  rain  below  freezing  point,  232 

"Local  air  drainage,"  259 

Lockyer,  Sir  Joseph  N.,  cited,  66n 

Lodge,  Sir  Oliver,  cited,  202 

Loess,  occurrence  of,  155 

London  Hospital  Medical  College,  experi- 
ments on  vitiated  air,  154 

Loomis,  E.,  cited,  217 

Low  ground  as  catchment  basins  for  slow- 
moving  air,  260 

Low  pressure  lane  and  flood  of  1913,  250 

Lows:  and  12-hour  pressure  fall,  88;  and 
trend  of  isotherms,  88;  deviation  of, 
and  unequal  pressure  distribution, 
88;  paths  of,  in  United  States,  [Fig. 
27]  91;  southern,  238 
movements  of:  and  pressure  change 
areas,  90;  empirical  rules  for,  88; 
individual,  67,  69 

Luminous  balls  from  storm  cloud,  191 


McAdie,  Alexander:  cited,  65,  66w,  110, 
202;  cloud  measurements  of,  110;  con- 
version scale,  [Fig.  9]  35;  new  unit 
of  pressure,  29;  observations  of  elec- 
tric potential,  8 

Madagascar  current,  72 

Magnetic  directions,  inconstancy  of,  55 

Magnetic  survey:  of  ocean,  first,  54;  on 
Halley's  voyage,  55 

Magnetism,  experiments  on,  by  Gay- 
Lussac,  10 

Mammato-cumuli,  181 

Manila  Observatory:  study  of  upper 
clouds  for  forecasting  purposes,  127 

Manson,  M.,  cited,  214 

Maps:  and  charts  of  winds,  early,  97; 
auxiliary,  of  pressure  and  tempera- 
ture changes,  89;  cloud  change,  89; 
first  rainfall,  217;  humidity  change, 
89;  synoptic,  basis  of  forecasting  by, 
90;  world,  of  wind  movement,  97 

March  storms,  89 

Maring  sunshine  recorder,  132w 

Mariotte,  E.:  cited,  6;  law  of  Boyle  and 
Mariotte,  39 

Marvin,  C.  F.,  cited,  9,  57w,  64n,  189w 

Mass:  of  atmosphere,  32;  of  earth,  32;  of 
oceans,  32;  standard  of,  32 

Matter,  unit  of,  25 

Maury,  Lieut.  M.  F.,  cited,  74;  work  on 
winds,  98 

Mayow,  John,  cited,  6 

Maze,  cited,  112 

Mechain,  P.  F.  A.,  cited,  25 

Mechanical  equivalent  of  heat,  38,  42; 
under  standard  gravity,  42 

Megabar,  30 

"Megalerg,"  42 

Megerg,  30,  42 

Meinardus,  W.,  cited,  70,  156,  232w,  236 

Meissner,  cited,  189 

Melting,  wind  an  important  factor  in,  229 

Mendenhall,  T.  E.,  cited,  201 

Mercury:  freezing  point  of,  288;  unit  of 
pressure,  27 

Meridian  ellipse,  perimeter  of,  32 

Meteorograph,  Dines's  light-weight  [Fig. 
6],  18;  kite,  development  of,  9 

Meteorology:  and  the  Accademia  del 
Cimento,  5;  Astro-,  1;  dawn  of,  1; 
dynamic  importance  of  terrestrial 
rotation  in,  60;  history,  of,  1,  24;  first 
international  system  of  observation, 
5;  in  time  of  Socrates,  2 
phenomena:  intensity,  in  symbols,  31; 
time  of  occurrence,  in  symbols,  31; 
values,  in  symbols,  31 

Meteors:  20;  phenomena,  indicating 
height  of  atmosphere,  19 

Meteorological  cycles  and  long  period  of 
tree  growth,  216,  217 

Meteorological  Office  of  Great  Britain, 
ocean  observations  of  Commander 
Hepworth,  published  by,  75 


308 


THE  INDEX 


Meteorological  Service  of  Canada,  bal- 
loon records  of,  19 

Meter:  25;  as  geographic  unit,  27;  con- 
version of  feet  into,  [Table  2],  279 

Metric  system,  27 

Miami  Street  Canal  Bridge,  Dayton, 
Ohio,  after  flood  of  1913,  [Fig.  97]  250 

Microbar,  30 

Microbes  and  dust,  154-166 

Middle  Atlantic  States,  snows  and  high 
winds  in,  90 

Miles,  conversion  of,  into  kilometers, 
[Tables  3,  4]  280;  interconversion  of 
statute  and  nautical,  [Table  5]  281; 
nautical  mile,  27 

Mill,  H.  R.,  cited,  218 

Millibar,  29,  30 

Milligal,  28n 

Millimeters,  conversion  of  inches  into, 
[Table  1]  279;  to  Kilobars  [Table  81 
284 

Minute,  27 

Mishnah,  2 

Mississippi  River,  drainage  basins  of,  246, 
[Fig.  95]  247 

Mistral  type  of  wind  105 

Mixing  ratio:  intensity  of  vertical  air 
movement,  determined  by,  268;  of 
dry  air  and  vapor,  143,  267;  record 
of,  important  in  frost  work,  267; 
vapor  content  determined  by,  268 

Mixture :  of  air  at  summit  level,  and  high 
night  temperature,  264;  of  air  masses, 
146:  that  cause  dissolution  of  vapor, 
153 

Mohn,  Professor  H.,  cited,  63 

Molecular  weights  of  atmospheric  gases, 
24 

Moller,  Max,  cited,  112 

Momentum,  angular,  law  of,  57;  linear,  in 
mingling  air  currents.  182;  unit  of,  26 

Monsoons:  97,  101;  and  deserts,  217;  and 
trades,  first  definite  knowledge  of, 
54;  first  European  knowledge  of,  2; 
meaning  of  term,  101;  pressure  fore- 
casts of  intensity  of,  102;  regions  of, 
101,  102;  summer,  101;  winds,  and 
rain,  217;  winter,  101 

Moon-light  record:  132;  during  eclipse, 
[Fig.  53]  133;  value  of,  133;  with 
cloud  intervals,  133 

Motion:  absolute,  of  radially  directed 
winds  at  surface  of  rotating  system, 
[Fig.  19]  62;  relation  of  law  of,  to 
pressure,  82 

Mountain:  breeze,  98;  influence  of,  on 
rainfall,  211;  passes,  forced  drafts  of, 
103;  slopes,  condensation  of  vapor 
on,  [Table  by  Pockels]  212;  winds 
dried  and  warmed  in  descending, 
[Fig.  60]  145 

Mount  Weather  Observatory,  Va. :  kites 
used  to  record  extreme  elevations,  18 

Mount  Whitney  Observatory:  records  of 
long  periods  of  cloudless  sky,  136 


Mount  Wilson  Observatory:  observations 
of  fluctuations  in  solar-constant 
values,  276 

Movement,  normal,  twenty-four  hour, 
for  storms,  [Table]  87 

Muhry,  A.,  cited,  112 

Muir,  John,  cited,  212 

Nadler,  G.,  cited,  47 

Nagel,  A.,  cited,  20 

Nares,  Sir  Geo.  S.,  cited,  8;  Nares' 
storm  kite,  8 

Nautical  mile:  27;  and  statute  mile,  inter- 
conversion  of,  [Table  5]  281 

Neon  in  atmosphere,  7;  percentage,  22 

Nephoscope,  use  of  in  cloud  observations, 
117,  123 

Neuhoff,  Georg  B.,  von:  adiabatic  dia- 
grams, 142,  144,  145,  268;  example 
for  mixing  ratio.  144 

Neuhoff-Hertz,  modified  adiabatic  dia- 
gram, [Fig.  57]  141 

Neumayer,  cited,   112 

Nevada,  data  on  snow  measure  from,  225 

New  Absolute  scale,  25,  36,  [Fig.  114]  289 

New  England  States:  cyclonic  advance 
on,  234;  ice  storms  of,  231;  snows 
and  high  winds  in,  90 

New  Orleans  storm  of  September,  1915: 
257;  change  of  winds  near  storm 
center,  [Fig.  103]  256;  pressure,  dur- 
ing storm,  [Fig.  102]  255 

Newton,  Sir  Isaac,  cited,  54;  formula  for 
velocity  of  sound,  37 

"Newton,"  28 

New  units,  29,  30 

New  unit  of  pressure,  29 

New  York  State  Commission  on  Ventila- 
tion, researches  of,  154w 

New  Zero,  36 

Night  cloudiness  recorder,  133 

Nimbus:  111,  115;  and  the  raindrop,  207 

Nipher,  Professor,  cited,  210 

Niton  in  atmosphere,  7 

Nitric  acid:  from  lightning  discharge,  198; 
in  air,  In 

Nitrogen;  and  argon,  percentage  in  air, 
Qn;  boiling  point,  287;  molecular 
weight,  24;  percentage  in  air,  22; 
peroxide  from  lightning  discharge, 
198 

"Norm,"  the,  218 

Normal  movement,  twenty-four  hour,  for 
storms,  [Table]  87 

Normal  values,  29 

North  American  hyperbar,  66 

North  Atlantic:  mean  temperature,  75; 
surface  temperatures,  relation  to 
weather,  75 

Northeast  current,  73,  74 

Northeast  wind  and  snowstorms,   229 

Northeasterly  icestorm:  conditions  for, 
type  of,  234,  237;  observations  of, 
235,  236 

Northern  Rocky  Mountain  storm,  87 


THE  INDEX 


309 


"Northers,"  of  California,   104 

North  Pacific  infrabar,  67 

North  Pacific  storms,  87 

Northwesterly  type  of  ice  storm,  236,  237 

Northwest  wind   and  snow  flurries,   229 

Nucleation  of  air  in  cyclonic  areas,  150; 
of  unfiltered  air,  variations  in,  161 

Nuclei:  in  supersaturated  air,  148;  neces- 
sity of,  in  air;  of  condensation,  dust 
as,  156;  persistent,  number  of,  150 

Obscuration  of  objects  by  dust,  157 

Observatories,  list  of,  in  international 
cooperation,  14,  15 

Ocean:  and  earth's  surface,  relative 
movements  of,  60;  area  of,  32;  circu- 
lation, effect  of  ice  flow  on,  71;  mass 
of,  32;  mean  density  of,  32;  radio- 
active content  of  atmosphere  over, 
201;  radioactive  content  of,  201; 
water,  heaping  of,  in  horse  latitudes, 
60 

currents:  70;  cold,  70;  effect  of,  on 
atmospheric  circulation,  70;  Hydro- 
graphic  Office  reports  on,  72;  like  air 
currents,  70;  southern,  72;  warm,  70 

Okada,  T.:  cited,  57n,  66n,  232n;  observa- 
tions of  raindrop,  240 

Ombroscope,  of  Fassig  and  Fergusson, 
211 

Onnes,  Professor  K.,  cited,  287 

Optical  phenomena,  164,  165,  166 

Oscillograph,  use  of,  for  lightning  records, 
190 

Oxygen,  boiling  point,  287;  molecular 
weight,  24;  percentage  in  air,  Qn,  22; 
separated  from  air  by  Lavoisier,  6 

Ozone  from  lightning  discharge  198 


Pacific  current:  72;  equatorial,  47 
Palazzo,     Professor      Luigi,     registering 

balloons  in  Zanzibar,  16 
Palmer,  A.  H.,  cited,  105,  164 
Pamperos,  of  Argentina,  105 
Parallax:  lunar,  32;  solar,  32 
Parapegmata     (peg     almanacs)     of     the 

Greeks,  1 
Paraselenae,  165 
Parhelia,  165 
Pascal,  Blaise:  3,  4,  5;  demonstration  of 

atmospheric  pressure,  5;  experiment, 

3,  [Fig.  1]  4 
"Pascal,"  28 
Paths    of   Galveston   hurricanes   of    1900 

and  1915,  [Fig.  101]  254;  of  highs  in 

the  United  States,  Van  Cleef's  chart, 

91 
Pavia     Observatory:     balloons    used    to 

record  extreme  elevations,  18 
Peak    discharges:    single-    and   multiple-, 

^190;  cause  of,  191 
Pelee,  eruption  of,  and  atmospheric  dust, 

163 

Pendulum  experiment  of  Foucault,  62 
Percentages,  volumes  of  gases,  22 


Perier,  M.,  investigations  of  atmospheric 
pressure,  5 

Pernter,  J.  M.,  cited,  146,  166 

Peru  current,  72 

Phenomenon,  atmospheric,  intensity  of, 
in  international  symbols,  31 

Philo  of  Byzantium,  cited,  2 

Photography  employed  in  cloud  measure- 
ments, 110;  first  used  from  kite,  by 
Archibald,  8 

Physical  Atlas  of  Berghaus,  217 

Pickering,  Professor  E.  C.:  Pole-star 
recorder,  133;  Pole-star  record, 
[Fig.  55]  134 

Piddington,  Henry,  cited,  77 

Pierce,  cited,  157w,  161 

Pilot  balloons:  16,  18;  and  pilot  planes, 
tests  by,  107 

Pilot  charts  of  trades,  issued  by  Hydro- 
graphic  Office,  98 

Pittsburgh's  annual  dust  deposit,    156 

Planes,  pilot,  tests  by,  107 

Planetary  circulation,  deflective  force  on, 
56;  Ferrel's  scheme  of,  56 

Planetary  pressure  distribution,  97 

Planetary  winds,  97,  98 

Plant  life,  temperature  of  cessation  of,  288 

Plotting  machine  for  measuring  cloud 
heights,  [Fig.  44]  126 

"Pluviometric  coefficient,"  218 

Pneumatics,  Hero's  book  on,  2 

Pockels,  Professor  F.,  cited,  212 

"Pockets,"  in  air,  107 

Poey,  A.,  cited,  112 

"Polar  bands,"  123 

Polar  cyclone,  63 

Polar  semi-diameter  of  earth,  32 

Pole-star:  record,  [Fig.  55]  134;  recorder, 
133 

Pomortzeff,  M.,  cited,  11 

Porta,  Sella  G.,  cited,  2 

Posidonius,  cited,  2,  3 

Post-Office,  Dayton,  Ohio,  after  flood  of 
March-April,  1913,  [Fig.  99]  252 

Potential,  electrical,  taken  from  balloon 
by  Glaisher  and  Coxwell,  10 

Potential  energy:  26;  transformed  into 
heat,  41 

Potential  gradient:  and  thunderstorm, 
199;  observations  of,  201 

Potential  values,  fluctuations  in,  during 
storms,  199 

Potsdam  Observatory:  cloud  observa- 
tions by  Sprung  and  Suring,  110 

Poundal,  26 

Power,  Horse-:  26,  27;  unit  of,  26 

Precipitation:  205-245;  and  thunder,  183; 
by  electric  discharge,  148;  causes  of, 
146;  conditions  for  maximum,  147; 
effected  by  cooling,  147;  factors 
affecting,  212;  halos  and,  164;  heavy, 
and  extensive  water  areas,  220; 
heavy,  and  track  of  cyclone,  220; 
in  dry  winter  month,  67;  in  flood  of 
1912,  [Fig.  98]  251;  in  foehn  wind, 


310 


THE  INDEX 


145;  in  Mississippi  basin  during 
flood  of  1912,  [Fig.  96]  249;  in  the 
United  States,  types  of  monthly  dis- 
tribution of,  [Fig.  82]  219;  in  wet 
winter  month,  68;  with  nuclei,  148 
Pressure:  areas,  and  abnormal  weather, 
66;  atmospheric  at  right  and  left  of 
prevailing  winds,  63;  auxuliary, 
maps,  89;  barometric,  factors  con- 
tributing to  increase  of,  during 
thunderstorm,  183;  base,  new,  29; 
change  areas  and  movements  of 
highs  and  lows,  90;  chart  of  non- 
periodic  changes,  89;  conversion 
table  of,  inches  to  kilobars,  283,  284; 
differences  in,  80;  fall,  12-hour  maxi- 
mum, and  storm  deviation,  88; 
fluctuations,  violent,  of  lightning 
discharge,  188;  forecasts  of  intensity 
of  monsoons,  102;  in  cyclone,  78; 
influence  of  stratosphere  and  tropo- 
sphere on,  80;  law  of  computation  of, 
and  of  application  of  laws  of  gases, 
82;  law  of  relation  of  motion  to,  82; 
lowest  on  Weather  Bureau  record, 
258;  of  aqueous  vapor,  determined 
by  Ferrel's  formula,  268;  of  satu- 
rated aqueous  vapor,  variations  of, 
[Table]  270;  reading  at  Galveston  at 
time  of  flood  of  1900,  252,  253; 
reading  at  Houston  during  hurricane 
of  August,  1915,  [Fig.  100]  253; 
reading  at  New  Orleans  during  storm 
of  September,  1915,  [Fig.  102]  255 
records  difficult,  151;  relations  of,  in 
strophic  balance,  83;  symbol  for,  83; 
temperature  and  conductivity,  rela- 
tion between,  201;  tilting  or  inequali- 
ties of,  107;  vertically,  directed,  182; 
and  volume,  temperature  effect  on, 
39;  volume  inversely  as,  39;  waves 
from  energy  of  lightning,  189 

distribution  of:  65,  106;  and  tempera- 
ture distribution,  relation  to  wind 
velocity,  81 ;  and  character  of  season, 
67,  68;  and  deflection  of  air  motion, 
56;  and  height  of  stratosphere,  50; 
and  ice-storm  inversions  of  tempera- 
ture, 234;  and  resulting  wind  direc- 
tion and  temperature,  104;  and  wind, 
law  of  equivalence  of,  84;  at  surface, 
and  convection,  86;  at  surface  con- 
trolled by  stratosphere,  86;  in  cy- 
clones and  anticyclones,  [Fig.  26]  81; 
operative,  81;  planetary,  87;  unequal, 
and  deviation  of  lows,  88 

gradient:  change  of,  with  height, 
formula  for,  84;  equation  for  change 
of,  85;  horizontal,  symbol  for,  83; 
maintenance  of,  81 

high:  after  thunderstorm;  181,  and 
downrush  of  air,  182;  and  inter- 
ference to  horizontal  flow,  182;  true 
arc,  temperature  of,  288 

sea-level,     and    surface    winds:    during 


dry  winter  months,  67,  [Fig.  20]  68; 
during  wet  winter  months,  67,  [Fig. 
21]  68 

units:  27;  American  (New)  and  Euro- 
pean (Old)  systems,  [Table]  30;  mer- 
cury, 27;  new,  29;  unit  of  force  of, 
27;  unit  of  weight  of.  27 

Priestley,  Joseph,  cited,  6 

Psychrometer,  aspiration,  on  manned 
balloons,  12 

Purga  type  of  wind,  105 

Pyrheliometers,  balloon,  experiments 
with,  18;  Callendar,  277;  use  of,  278 

Pyronometer,  use  of,  278 

Radiation:  alpha,  in  upper  atmosphere, 
278;  and  stratosphere,  50;  and  sun- 
spot  numbers,  276;  a  function  of 
absolute  temperature,  260;  energy, 
source  of,  273;  free,  interference  of, 
by  condensed  vapor,  261;  from  air, 
slow,  264;  from  earth,  rapid,  264; 
nocturnal,  244;  point  of  upper  clouds, 
117;  radio-active,  in  upper  atmos- 
phere, 278;  shown  by  ionization  in 
upper  atmosphere,  278;  solar  con- 
stant of,  275;  temperature,  mini- 
mum, 51;  total,  at  midday,  277; 
effects  of:  260;  in  lower  and  upper 
levels,  53;  loss  and  gain  of  heat,  167 
solar:  and  atmosphere,  absorption  and 
reflection,  275;  and  sunspot  numbers, 
276;  and  volcanic  eruptions,  277; 
causes  of  variation  in  intensity  of, 
276;  Dorno's  spectrum  records  of, 
273;  energy  of  sun,  measure  of,  273; 
in  clear  sky  at  midday,  277;  intensity 
of,  records  of,  273;  law  of  energy  of, 
278;  measurements  of,  18;  recent 
measurements,  278 

Radio-active  cosmical  dust :  in  upper 
atmosphere,  278;  content  of  atmos- 
phere: over  land,  201;  over  ocean, 
201;  content  of  ocean,  investigation 
of,  201 

Radioactivity,  observations  of,  201 

Radius:  angular,  symbol  for,  83;  of  earth, 
equatorial,  31;  of  earth,  symbol  for, 
83 

Rain:  205;  and  nucleation,  161;  and  trade 
or  monsoon  winds,  217;  area,  move- 
ment on  Pacific  coast,  69;  current 
densities  of  electricity  in,  169; 
electrical  charge  carried  by,  169; 
excess  of  positive  electricity  in,  169; 
gushes,  183;  in  Hertz'  classification 
of  condensation,  140;  intensity  of, 
and  charge,  170;  international  sym- 
.  bol  for,  31;  liquid,  below  freezing 
point,  232;  periods  of  positive  and 
negative  electricity  in,  169;  posi- 
tively charged,  processes  that  make, 
171;  quantities  of  positive  and  nega- 
tive electricity  in,  169;  -reporting 
stations  in  Korea,  209;  stage,  cooling 


THE  INDEX 


311 


Rain,  cont'd:  of  air  in,  142;  temperature 
in  ice  storms,  231:  see  also  Cloud; 
Condensation;  Electricity;  Storm; 
Thunderstorm 

gauge:  207;  British,  209;  Fergusson, 
210;  location,  210;  place  of  exposure 
should  be  permanent,  211;  early 
Korean,  207;  oldest,  [Fig.  80]  208; 
Snowdon,  209;  tipping-bucket,  169, 
210;  U.  S.  Weather  Bureau,  210 
probability:  and  alto-cumulus,  130; 
and  alto-stratus,  129;  and  cirro- 
stratus,  129 

Rainbows:  166;  international  symbol  for, 
31 

Raindrop:  and  cirro-cumulus,  cirro- 
stratus,  low  cumulus,  nimbus,  207; 
and  lightning,  [Table]  207;  coales- 
cence and  disruption  of,  in  cloud,  171; 
frozen,  sizes,  232;  not  pure  water, 
205;  process  that  makes,  205;  rate 
of  cooling,  240;  relation  of,  to  storm 
conditions,  206;  size,  205;  variation 
of,  with  storm  locus,  206;  velocity  of 
thunder,  lightning  and,  184;  weight, 
205 

Rainfall:  and  snowfall,  charge  per  unit 
in,  170;  and  snowfall,  vertical  cur- 
rents in,  170;  as  cooling  agent  of  a 
thunderstorm,  177;  charting,  220; 
diminution  poleward,  217;  early 
measurements,  2;  east  of  Sierra 
Crests,  213;  "equipluves,"  218;  ex- 
ceptional, at  Baguio,  P.  I.,  214;  fre- 
quency of  negative  electricity  in 
different  periods,  170;  heaviest  ever 
recorded,  shown  by  Baguio,  (P.  I  ) 
Weather  Bureau,  [Fig.  81]  215; 
heaviest  in  United  States,  214; 
heavy,  with  electrical  discharge,  207; 
influence  of,  on  htirricanes,  88;  in 
California,  213,  214;  in  equatorial 
regions,  217;  in  hearts  of  continents, 
poleward,  218;  intensity,  and  charge 
decrease,  170;  list  of  heaviest,  216; 
long  period  records,  211;  normal,  218; 
on  coastal  lands,  poleward,  217;  on 
eastern  coasts,  217;  on  mountain 
lands,  poleward,  218;  on  mountains 
in  South  Africa,  214;  periodicity 
shown  by  annular  growth  of  trees, 
214;  potential  gradient  in  different 
periods  of,  170;  "  pluviometric  coeffi- 
cient," 218;  prompt  measurement  of, 
211;  rate  of,  and  electricity,  169; 
relative  humidity  in,  177;  "smear," 
218,  220;  "splash,"  218;  upon  plains 
of  northern  India,  213;  velocity  and 
broken  raindrops,  171 
distribution:  217;  cyclonic  unit  for, 
220;  in  West  Indian  hurricane  of 
1915,  [Fig.  104]  257 

topography  and:  220;  areas  of  exces- 
sive, 218;  elevation  of  maximum, 
212;  influence  of  mountains  on,  211; 


variation  of,  with  altitude  in  moun- 
tainous countries,  211:  see  also 
Cloud;  Electricity;  Storm;  Thunder- 
storm 

Ramsay,  Sir  William,  cited,  6,  7 

Rauhreif,  233 

Rayleigh,  Lord,  cited,  7;  color  and 
atmospheric  dust,  163 

Reaumur's  scale,  35 

Reciprocal  of  flattening  at  poles,  32 

Recorder:  night  cloudiness,  133;  Pole- 
star,  133;  sunshine,  131;  sunshine, 
Campbell-Stokes,  132;  sunshine, 
Maring,  132w 

Records  of  air  motion  difficult,  151;  of 
moonlight,  and  night  clouds,  133; 
of  state  of  sky,  hours  for,  133 

Redfield,  W.  C.,  cited,  77 

Red  sunsets,  163 

Reed,  M.  G.,  cited,  220 

Reflection  and  solar  radiation,  275 

Region  of  maximum  change,  90 

Regions  of  heaviest  snowfall,  229 

Reid,  Wm.,  cited,  77 

Relative  motion  of  radially  directed  winds 
at  surface  of  rotating  system,  [Fig. 
18]  62 

Relative  movements  of  ocean  and  earth's 
surface,  60 

Riccioli,  G.  B.,  cited,  21;  cloud  measure- 
ments of,  110 

Richards,  Professor  T.  W.,  cited,  30 

Riggenbach,  Albert,  cited,  112;  cloud 
atlas  of,  113 

Rime  (hoarfrost),  230 

"Rime":  official  use  of  term,  233w 

Rises,  90 

Robertson,  E.  G.  R.,  cited,  10 

Robinson  anemometers,  record  of,   105 

Rocket  lightning,  192,  193 

"Rolling"  of  thunder,  188,  189 

Roll  scud  and  hail,  186 

Rotation:  air  movement  relative  to,   61; 
gradient  and  curvature  gradient  in 
cyclones   and   anticyclones,    64;   ter- 
restrial, and  air  movements,  54 
cyclonic:  determination  of  velocity  of, 
62;  elevation  and  increase  of  velocity 
of,  62 ;  level  of  maximum,  62 ;  of  air,  62 
earth's:  deflecting  force  of,  56;  effect  of, 
on  atmosphere,  54 

Rotch,  A.  Lawrence;  cited,  3n,  8n,  11,  12, 
15,  48,  49,  105,  167;  cloud  measure- 
ments, 110;  diagram  of  height  of 
stratosphere  with  latitude,  [Fig.  11]  51 

Rotenturm  wind,  104 

Royal  Meteorological  Society:  Cave's 
investigations  presented  to,  7,  8; 
Hellmann's  citations  read  before,  1 

Royal  Society  of  Edinburgh,  Transactions 
of,  with  Aitken's  observations  on 
atmospheric  dust,  156,  IQQn 

Russell,  Rollo,  cited,  242 

Russell,  S.  C.,  classification  of  clouds,  111, 
112n2,  134 


312 


THE  INDEX 


Russian  Academy,  ascents  by  Robertson 

from  Petrograd,  directed  by,  10 
Rutherford,  Ernest,  cited,  6 


Sacramento,  rainfall  records,  213 

St.  Elmo's  fire,  193 

Salt  content  of  atmosphere,  164 

Sandstrom,  J.  W.,  quoted,  59,  60,  61 

"Santa  Anas,"  of  southern  California, 
104,  144 

Saturation:  aqueous  vapor  at,  [Table  13] 
292;  aqueous  vapor,  pressure  of, 
variations  of,  [Table]  270;  deficit, 
and  rate  of  evaporation,  241;  deficit 
recorder,  [Fig.  110]  268;  deficit  re- 
corder, the  hygrograph,  266;  law  of, 
83;  vapor  of,  and  aqueous  vapor, 
[Table  12]  291 

Scale,  temperature:  33-36;  absolute,  29, 
35,  489;  Absolute  centigrade,  35; 
Absolute,  critical  temperatures  on, 
287,  288;  Absolute  energetic,  36; 
Celsius,  35;  Centigrade,  35,  489; 
chart  of  four,  [Fig.  114]  289;  con- 
venient conversion,  [Fig.  9]  35; 
Fahrenheit,  35,  489;  Linnaeus'  modifi- 
cation, 35;  New  Absolute,  25,  36,  489; 
Reaumur's,  35:  thermodynamic,  36 
wind:  Beaufort,  290;  equivalent  values 
for,  290;  formula  for,  290;  [Table  11] 
290 

Scheele,  C.  W.,  cited,  6 

Schmidt,  Dr.  Wilhelm,  studies  on  light- 
ning, 187,  188,  189,  234w 

Scoresby,  Wm.  Jr.,  classification  of  snow 
crystals,  221 

Scott,  Captain,  R.  F.,  cited,  15,  47;  lowest 
temperature  recorded  by,  288 

"Scud,"  115 

Sea  breeze,  97,  98 

Season:  character  of,  and  persistency  of 
hyperbars,  66;  character  of,  and 
pressure  distribution,  67;  effect  of, 
on  stratosphere,  50 

Seasonal  winds,  cause  of,  98 

Second:  26,  27;  mean  solar,  26 

Secondaries,  formation  of,  90;  move- 
ments of,  90;  tornadoes  a  type  of,  90 

Shaw,  Sir  (William)  Napier,  cited,  29, 
63,  64,  77,  80,  90,  107,  272;  Shaw's 
equations,  80;  five  laws  of  atmos- 
pheric motion,  82,  83;  formula  for 
values  of  gas  constant,  40;  geostro- 
phic  wind,  84;  principle  of  strophic 
balance,  83 

Sheet  lightning,  193;  see  also  Lightning 

"Shock  wave"  in  thunder,  188 

Shooting  stars,  meteors,  bolides,  20 

Sierra,  cloudless  periods  in,  136 

Sigsfield,  cited,  153 

Silver,  melting  point  of,  288 

"Silver  thaw,"  230 

Simla  Observatory:    see  Indian  Meteoro- 


logical    Department;     Dr.     C.     G. 
Simpson 

Simoon,  104 

Simpson,  Dr.  C.  G.,  cited,  15,  148;  atmos- 
pheric electricity,  168,  200;  measure- 
ments of  aurora,  278;  observations 
at  Simla,  168;  records,  169;  theory  on 
origin  of  electricity  in  thunderstorms, 
172n 

Singer,  Karl,  cited,  112 

Sirocco:  dust  in  Europe,  156;  of  southern 
Italy  and  Greece,  104 

Sivel,  H.  T.,  cited,  10 

Skinner,  S.,  drosometer,  245 

Sky:  hours  for  recording  state  of,  133; 
radiation,  diffuse,  277;  why  it  is  blue 
163 

Sleet:  230,  231,  244;  international  symbol 
for,  31;  official  use  of  term,  233n; 
structure  of,  indicating  conditions  of 
air  stratification,  232 

"Smear,"  218,  220 

Smith,  J.  W.,  insurance  statistics  on 
lightning,  202 

Smithsonian  Institution:  experiments  of 
Astrophysical  Observatory  to  obtain 
measurements  of  solar  radiation,  18; 
publication  of  book  on  Winds  of  the 
Globe,  by  Professor  J.  H.  Coffin,  97 

Smoke  Abatement  League  of  Cincinnati, 
observations  of  city  dust,  156 

Snow:  221-229;  and  gradient  decrease, 
140;  and  high  winds  in  Middle  Atlan- 
tic and  New  England  states,  90; 
and  winter  cyclones,  229;  "banners" 
in  the  Sierra.  212;  computing  rate  of 
melting,  228;  cover  in  mountains, 
method  of  studying,  [Fig.  87]  228; 
cover,  water  supply  from,  225; 
crystallization,  221;  data,  losses 
through  evaporation,  225;  depth  at 
Summit,  Cal.,  [Fig.  86]  227;  distrib- 
ution, effect  of  topography  on,  224; 
drifting,  international  symbol  for, 
31;  electrification  of,  170;  forecasts 
of  water  supply  from,  226;  frozen 
precipitation,  230,  231;  in  storms, 
distribution,  224;  international  sym- 
bol for,  31;  measure  in  United  States, 
data  on,  225;  sampler,  223;  sampler 
by  Church,  225;  -stage,  cooling  of  air 
in,  142;  surrounding  country  more 
than  half  under,  international  sym- 
bol for,  31;  surveys  for  prediction  of 
floods  and  water  resources,  225; 
water  equivalent  of,  223 
crystals:  221,  [Fig.  83]  222,  [Fig.  84] 
223,  [Fig.  85]  224;  and  temperature 
of  air,  221;  types  of,  221;  types  of, 
in  cold  snowfalls,  221 
economic  importance:  225;  depth,  to 
mining  and  irrigating  operations, 
226;  depth,  to  power  plants,  226 

Snowbank,  variations  in  density,  223 

Snowdon  rain  gauge,  209 


THE   INDEX 


313 


Snowfall:  and  rainfall,  charge  per  unit 
in,  170;  and  rainfall,  vertical  currents 
in,  170;  heaviest,  regions  of,  229; 
measurements  of,  223;  records,  226; 
seasonal  curve,  225;  total.  225 

Snowflakes,  small,  [Fig.  84]  223;  structure 
of,  [Fig.  85]  224;  see  o/so[Snow  crystals 

Snowstorms:  fluctuations  in  potential 
values  during,  199;  greatest,  from 
northeast,  229;  voltage  of  air  during, 
[Fig.  79]  201 

Socrates,  cited,  2 

Solano  (east  wind)  104 

Solar  constant:  18;  of  radiation,  275; 
values,  fluctuations  of,  276,  277 

Solar  influences,  273-278 

Soley,  Lieutenant,  cited,  72,  74 

Soot:  deposit  in  Leeds,  England,  156; 
deposit  in  Pittsburgh,  156 

Sound:  Laplace'  formula  for  velocity  of, 
37;  Newton's  formula  for  velocity  of, 
37;  velocity  of  propagation  of,  37 

Soundings,  balloon,  record  of,  at  Uccle, 
[Table]  47 

South  Africa:  hyperbar  in,  65;  rainfall  on 
mountains  of,  214 

South  Atlantic  currents,  direction  of, 
shown  by  drifting  ice,  75;  type  of 
storms,  87 

South  Pacific:  hyperbar,  65;  storm,  87 

Southerly  type  of  ice  storm,  234 

Southern  ocean  current,  72;  velocity,  73 

Southwell,  cited,  6 

Specific  conductivity,  observations  of,  202 

Specific  heat:  34;  at  constant  pressure, 
44;  of  air,  34,  37;  of  air  at  constant 
pressure,  34;  formula,  43;  at  con- 
stant volume,  34,  43;  of  water  vapor, 
at  constant  pressure,  34;  of  water 
vapor  at  constant  volume,  34 

"Specific  humidity,"  267 

Spectrum:  lightning,  [Fig.  75]  194, 
[Fig.  76]  195,  [Fig.  77]  196;  light- 
ning, lines  in,  [Table]  196,  197; 
solar,  variations  of  ultra-violet  radia- 
tion of,  274;  solar,  visible,  by  Dorno, 
273;  ultra-violet,  indicating  seasonal 
heating  effects  of  sun,  273 

Spinelli,  J.  E.  Croce,  cited,  10 

Spitsbergen  current,  73 

"Splash,"  218 

Spring  sun,  heat  of,  273 

Sprung,  A.,  cited,  63;  cloud  observations 
of,  110 

Squall  cloud,  179 

Stage,  dry,  cooling  of  air  in,  142 

Standard  gravity,  42 

Star  recorder,  133 

Statute  and  nautical  miles,  interconver- 
sion  of,  [Table  5]  281 

Steadworthy,  A.,  quoted,  194;  lines  in 
spectrum  of  lightning,  194-197, 
[Table  ]196 

Steam,  British  thermal  units  absorbed  in 
changing  water  to,  271 


Stearns,  Prof.  W.  B.,  weather  observer, 
service  of,  in  giving  timely  warning, 
256 

Steinmetz,  Andrew,  cited,  191 

Stolberg,  August,  cited,  17 

Stormer,  Carl  O.,  aurora  measurements, 
278 

Storms,  Alberta  type,  87;  and  cirrus,  130; 
center,  78;  centers,  finding,  220; 
Colorado  type,  87;  Central  type,  87; 
circulations,  typical,  Bigelow's  report 
on,  110;  cloud  formations  in  advance 
of,  [Fig.  50]  130;  cloud,  luminous 
balls  from,  191;  East  Gulf  type,  87; 
forecasting,  87-96;  frequency  and 
floods,  relation  between,  246,  248; 
intensity  and  direction  indicated  by 
rapid  temperature  rises,  89;  intensity 
and  horizontal  temperature  gradi- 
ents, 89;  kite  of  Nares,  8;  movements 
and  cirrus,  131;  movement,  types  of, 
preceding  frosts,  264;  March,  89; 
normal  twenty-four  hour  movement, 
[Table]  87;  North  Pacific  type,  87; 
Northern  Rocky  Mountain  type, 
87;  notable,  and  floods,  246-258;  of 
Apalachicola,  1915,  257;  of  con- 
tinental types,  87;  of  1900  and 
August,  1915:  comparison  by  Frank- 
enfield,  254,  255;  and  highs,  over 
Great  Lakes,  88;  over  Lake  region,  90; 
paths  in  the  United  States,  Van 
Cleef's  chart,  91;  prime  movers  of, 
79;  source  of  energy  of,  78;  South 
Atlantic  type,  87;  South  Pacific  type, 
87;  Texas  type,  87;  thunder-,  inter- 
national symbol  for,  31;  tracks,  78; 
types  of,  87;  velocity  of,  184;  types 
of  winds,  97;  weather  bureau  at 
Galveston,  warnings  preceding,  256; 
West  Indian  type,  87:  see  also  Hurri- 
canes, Rain,  Thunderstorms 

Strata,  unlike,  close  proximity  of  in 
frost  conditions,  268 

Stratification  of  lower  air,  107 

Stratiforms,  [Clayton's  chart]  119 

Strato-cumulus,  [Fig.  36]  114;  [Fig.  41] 
122;  123;  definition  of,  115 

Stratosphere:  46,  83;  actual  temperature 
of,  50;  and  troposphere,  46-53; 
annual  variation  in  height  of,  [Fig. 
12]  52;  base,  52;  base  in  equatorial 
regions,  and  lowest  recorded  tem- 
perature, 18;  behavior  of  cumulus 
cloud  near,  52;  direction  of  convec- 
tion in,  46;  distribution  of,  85;  due 
to  radiation,  50;  explanation,  50; 
in  tropics,  51;  influence  of,  on  surface 
pressure,  80,  85;  influence  of  pressure 
distribution  on  height  of,  50;  mean 
heights  and  temperatures  at  base  of, 
49,  [Table]  50;  rapid  falling  off  of 
winds  in,  86;  Rotch's  diagram  of 
height  with  latitude,  [Fig.  11]  51; 
seasonal  variations  in  height  of,  50; 


314 


THE  INDEX 


Stratosphere  cont'd:  strongest  current 
107;  temperature  and  height  below, 
in  summer  and  winter,  19;  variation 
in  level  of,  19,  46,  50,  [Table]  52; 
variation  with  latitude,  48 

Stratus,  111,  112,  117,  [Fig.  41]  122,  151 

Stream  flow,  snow  surveys  for  studies  in, 
225 

Strophic  balance:  equations  for,  84; 
principle  of,  83 

Structure:  atmospheric,  Cave's  five  types 
of,  106;  thermal,  of  air,  and  velocity 
increase  aloft,  82;  thermal,  general 
characteristic  of,  82 

Stuntz,  S.  C.,  cited,  155 

Sublimation:  see  Latent  heat  of  vapori- 
zation 

Sulphur  dioxide  in  air,  162 

Sulphuric  acid  in  air,  7n 

Summer  sun,  heat  of,  273 

Summit,  rainfall  records  at,  213 

Summit  Observatory:  curve  showing 
mean  depth  of  snow,  by  Prof.  Le 
Conte,  226-229 

Sun:  a  variable  star,  277;  and  earth,  rela- 
tive sizes  of,  [Fig.  112]  274;  diameter, 
32;  direct  photograph  of,  [Fig.  112] 
274;  heat  received  at  different  sea- 
sons, 273,  274;  temperature,  288; 
ultra-violet  rays  at  different  seasons, 
273;  visible  spectrum  of,  273 

Sunset    effects,    [Figs.    32,    33,    34]    109 

Sunsets,  why  they  are  red,  163 

Sunshine:  causes  of  variation,  275; 
duration  of,  [Tables]  275,  276; 
nuclei,  and  fogs,  162;  recording,  131; 
variation  in,  275;  variations  with 
longitude,  275 

recorder:  Campbell-Stokes,  [Fig.  52] 
132;  Maring,  132w;  photometric,  132; 
therm  ometric,  132 

Sunspot:  cycle  and  tree-growth  period, 
217;  numbers  rnd  solar  radiation, 
276;  numbers,  Wolff,  276 

Supan,  Alexander,  cited,  217 

Supersaturation,  148 

Surface:  air,  flow  of,  and  frost  forecast, 
259;  of  earth,  mean  density,  32; 
temperature  of  atmosphere,  22 

Suring,  R.  J.,  cited,  11;  cloud  observa- 
tions of,  110 

Surrounding  country  more  than  half  under 
snow,  international  symbol  for,  31 

Swann,  cited,  201 

Sweetland,  cloud  measurements  of,    110 

Symbols:  for  factors  in  strophic  balance, 
83,      84;      meteorological,      interna- 
tional, 31;  units  and,  25-32 
letters:   international   of   cloud    names, 
113,  115,  117,  for  geodetic  data,  32 

Symons,  G.  J.,  founder  of  British  Rainfall 
Organization,  209 

Synoptic  maps,  basis  of  forecasting  by,  90 

System,  international,  of  cloud  classifica- 
tion, 113 


Taal,  eruption  of,  and  atmospheric  dust, 
163 

"Tablecloth,"  151,  212 

Talman,  Professor  C.  F.,  cited,  31w 

Teisserenc  de  Bort,  Leon,  cited,  9.  11, 
46,  48,  65,  112,  113 

Telephone,  use  of,  between  observing 
stations,  110 

Temperature:  abnormal,  periods  of,  65; 
absolute,  radiation  function  of,  260; 
absolute,  system  of,  33;  actual,  of 
stratosphere,  50;  and  heights,  mean, 
at  base  of  stratosphere,  [Table]  50; 
and  pressure  distribution,  computed 
from  wind  velocity,  81;  and  wind 
direction  resulting  from  pressure 
distribution,  104;  and  velocities  in 
cyclones,  diagrams  of,  79,  80,  81; 
at  cirrus  level,  [Table]  51;  at  upper 
fog  layer,  153;  auxiliary  maps,  89; 
changes  in  thunderstorm,  183;  con- 
ductivity and  pressure,  201;  con- 
stant, height  of,  22;  conversion  of, 
[Table]  285;  critical,  on  absolute 
scale,  287,  288;  decrease,  uniformity 
of,  22;  difference,  in  cloud,  174; 
discussion,  33;  distribution  in  cy- 
clones and  anticyclones,  62,  [Fig.  25] 
80;  during  flood  of  1913,  252;  effect 
on  volume  and  pressure,  39;  equilib- 
rium, convective,  50;  for  disappear- 
ance of  electrical  resistance,  287; 
forecast  by  cirrus,  127;  freezing,  at 
high  level,  185;  freezing,  of  objects  in 
ice  storms,  137;  high,  cause  of,  on 
summit,  at  night,  264;  low,  in  upper 
tropical  troposphere,  51;  low,  ob- 
tained by  Rotch,  at  base  of  strato- 
sphere, 288;  lowest  recorded,  18,  48, 
[Table]  49,  288;  lowest,  recorded  by 
Scott  in  south  polar  regions,  288; 
lowest,  region  of,  and  heaviest  frost, 
270;  method  showing  diminution  of, 
with  altitude,  142;  minimum  radia- 
tion, 51;  minus,  conversion  of,  [Table 
10]  286,  287;  monthly  values  of, 
[Fig.  10]  48;  North  Atlantic  mean, 
75;  North  Atlantic  surface,  relation 
to  weather,  75;  of  Antarctica  land 
mass,  70;  of  cumuli,  153;  of  dry  air, 
calories  required  to  raise,  137;  of 
front  and  back  of  cyclones,  and 
anticyclones,  78;  of  space,  effective, 
287;  of  sun,  288;  preceding  fog  dissi- 
pation, 152;  rapid  rise  of  at  valley 
level  after  sunrise,  264;  records  diffi- 
cult, 151;  record  during  ice  storm. 
[Fig.  89]  235;  [Fig.  90]  239;  region  of 
constant,  and  convection  limits,  23, 
24;  relations  of,  in  strophic  balance, 
83;  scales,  33-36;  scales,  four, 
charted,  [Fig.  114]  289;  scales,  varie- 
ties of,  35;  see  Scales;  sea,  and  path 
of  air  during  fog  off  Newfoundland, 
[Fig.  61]  149;  sea  fog,  150;  slope  of, 


THE  INDEX 


315 


Temperature,  cont'd:  81,  82;  seasonal,  in 
stratosphere,  19;  sudden  changes  of, 
in  icestorm,  238;  surface,  22;  symbol 
for,  83;  system,  absolute,  33;  upper 
air,  at  Batavia,  lowest,  18 
fall:  adiabatic  rate,  264,  265,  [Table] 
138;  rate,  recorded  in  balloon  ascents, 
10;  regular,  above  equator,  48; 
gradients:  actual,  44;  approximate, 
[Tables]  44,  45;  and  formation  of 
clouds,  53;  change  of  sign  in,  53; 
controlling  cloud  movements,  127; 
effect  of  season  and  latitude  on,  46; 
equilibrium,  stable,  condition  for, 
138;  horizontal,  and  storm  intensity, 
89;  horizontal,  symbol  for,  83; 
level  of  cessation,  46;  level  of  rever- 
sion, 46;  negative,  condition  for,  139; 
per  thousand  meters,  45;  within  and 
without  cumulus  clouds,  [Fig.  64]  173 
—  adiabatic,  at  different  heights, 
[Table]  38;  formula,  37;  of  saturated 
air,  [Table]  138;  variations  of,  138 
inversions  of  ice  storm,  234,  239;  in- 
version on  sea  fog,  150;  inversions  by 
vertical  movements  in  air,  259; 
inversions,  records  of,  262 
rise:  and  fall  of  humidity,  265;  at  lowest 
level,  264;  rapid,  and  direction  of 
storm,  89;  rapid,  and  intensity  of 
storm,  89;  sharp,  indications  of,  89 

Terrestrial  Magnetism,  Department  of,  at 
Carnegie  Institution:  radioactive 
content  of  ocean,  201 

Terrestrial  rotation:  and  air  movements, 
54;  importance  in  dynamic  meteor- 
ology, 60 

Terrestrial  winds,  98 

Texas  storms,  87 

Thawing  and  electrification,  170 

Theodolites,  self-recording,  used  in  Aus- 
tralian soundings,  18 

Theophrastus,  cited,  2 

Therm,  42 

Thermal  unit:  34;  British,  270 

Thermodynamic  scale,  36 

Thermodynamics  of  the  atmosphere,  37- 
45;  146 

Thermograph,  266;  care  in  use  of,  269; 
curves,  showing  sudden  changes  dur- 
ing ice  storm,  239,  [Fig.  90]  239,  240; 
dry  and  wet,  location  for,  269;  for 
dry  and  wet  bulb  readings,  (Foxboro 
type)  [Fig.  Ill]  269;  record  showing 
types  of  inversions,  [Fig.  105]  261 

Thermoisopleths,  75 

Thermometers:  Assmann  aspirated,  11; 
constant-volume,  36;  Florentine, 
first  in  England,  6;  hydrogen-gas, 
36;  self-recording,  11;  sling,  11; 
standardization  of,  for  balloon  ex- 
plorations, 12;  wet  bulb,  146 

Thermoscope,  of  Philo  of  Byzantium  and 
Hero  of  Alexandria,  2 

Thiessen,  cited,  225 


Thunder:  and  precipitation,  183;  "claps" 
of,  189;  claps,  prolonged,  wave 
reflection  from  "interfaces,"  188; 
distant,  international  symbol  for,  31; 
inaudible  waves  in,  188;  intervals  of, 
189;  irregularity  of  waves  in,  187; 
lightning  and  raindrops,  velocity  of, 
184;  "rolling"  of,  188,  189;  "shock 
waves"  in,  188;  "tone"  rare  in,  187; 
tremendous  energy  of,  189;  violent 
waves  of,  in  irregular  series,  188 

Thunderbolts,  stalled,  and  ball  lightning, 
193 

Thundercloud,  and  rapid  vertical  convec- 
tion, 174;  brush  discharge  from.  193; 
coronal  discharge  from,  193;  see 
Cloud;  Electricity;  Rain;  Storm; 
Thunder 

Thunderstorms:  96;  air  current  in,  171; 
and  atmospheric  descent  in  rainfall, 
178;  and  potential  gradient,  199; 
barometric  change  in,  181;  cause  of 
turbulence  in,  173,  conditions  favor- 
able to,  176;  clouds,  turbulence  in, 
[Figs.  65,  66]  175;  cold  downdraft  of 
air  in,  178;  descent  of  potentially 
cold  air  as  cooling  agent,  176;  essen- 
tials to  genesis  of,  183;  evaporation 
during  rainfall  as  cooling  agent,  176, 
177;  experiments  for  electrical  effects 
of,  170;  factors  contributing  to  in- 
crease of  barometric  pressure  during 
thunderstorm,  183;  fluctuations  in 
potential  values  during,  199;  hail  in, 
184;  high  humidity  on  day  of,  183; 
high  pressure  after,  181;  hours  of 
maximum  frequency,  176;  Hum- 
phrey's theory  on  cooling  of,  176; 
ideal  cross-section  of  typical,  [Fig. 
68]  181;  international  symbol  for,  31; 
in  the  making,  [Fig.  67]  180;  months 
of  greatest  frequency,  176;  origin  of 
electricity  in,  171,  172;  phenomena, 
167;  rainfall,  as  cooling  agent,  176, 
177;  relative  humidity  in,  177; 
temperature  changes  in,  183;  under- 
running  cold  current,  178;  velocity 
of  in  Europe  and  in  United  States, 
184;  voltage  of  air  during,  [Fig.  78] 
200;  wave  motion  during,  187;  see 
also  Clouds;  Electricity;  Rain;  Storm 

Time,  unit  of,  26 

Tin,  melting  point  of,  288 

Tipping  bucket  rain  gauge,  210 

Tissandier,  Gaston,  cited,  10,  11,  112 

Toepler,  quoted,  193 

"Tone"  in  thunder,  187 

Topography,  effect  of,  on  ice  storm,  233; 
governing  rainfall,  221;  influence  on 
winds,  102;  local,  effect  of,  on  snow 
distribution,  224 

Tornadoes:  90;  as  example  of  vortex 
motion,  93;  as  secondary  and  local- 
ized vortices,  90;  at  Lawrence,  Mass., 
[Fig.  29]  93;  cause  of,  79,  92;  cause 


316 


THE   INDEX 


Tornadoes,  cont'd:  of  destructive  power, 
92;  characteristic  feature,  92;  effects 
of  activity,  92;  original  use  of  word, 
92;  paths  of,  90;  velocity  of  air  in 
vortex  of,  93 

Torricelli,  Evangelista,  cited,  3;"  Torri- 
celli's  tube":  see  Barometer 

Tower  of  the  winds  at  Athens,  1 

Toynbee,  Captain  Henry,  cited,  112 

Trabert,  Wilhehn,  cited,  50 

Tracy,  Charles,  earth's  deflective  effect 
on  air,  55w2 

Trades:  97,  98;  and  dry  zones,  217;  and 
monsoons,  first  definite  knowledge 
of,  54;  and  rain,  217;  and  trans- 
Atlantic  flights,  106;  counter-,  100; 
Hadley's  theory  of,  55;  in  problem  of 
trans- Atlantic  flight,  106;  pilot  charts 
of,  issued  by  Hydrographic  Office,  98; 
spring,  of  Atlantic,  98;  spring,  of 
Indian  Ocean,  99;  spring,  of  the 
Pacific,  98;  wind  direction  above,  48; 
winter,  of  Atlantic,  99;  winter,  of 
Indian  Ocean,  100;  winter,  of  Pacific, 

qq 
yy 

Trees,  liability  of,  to  lightning  stroke, 
203;  percentages,  struck  by  light- 
ning, [Table]  203 

Tree-growth  period:  and  sunspot  cycle, 
217;  long,  and  meteorological  cycles, 
217 

Troposphere,  46,  83;  influence  of,  85; 
influence  on  pressure,  80;  rate  of 
change  of  velocity  in,  86;  tropical, 
upper,  low  temperatures  in,  51 

Turbulence  in  Thunderstorm  c.ouds, 
[Figs.  65,  66]  175 

Twilight  arch,  and  upper  limit  of  the 
atmosphere,  20 

Types:  continental,  storms  of,  87;  five,  of 
atmospheric  structure,  106;  of  inver- 
sions, thermograph  record  showing, 
[Fig.  105]  261;  of  storms,  87;  of 
winds,  97 

Typhoons  of  China  Sea,  77 

Typhoon  type  as  forecast,   125 

Uccle  Observatory,  record  of  soundings 
made  by,  [Table]  47 

"Ulloa's  ring,"  166 

Ultra-violet  radiation:  and  "glacier 
burn,"  274;  and  heat,  274;  at  differ- 
ent seasons,  273,  274;  of  solar  spec- 
trum, variations,  273 

Underrunning  cold  current  in  thunder- 
storm, 178 

United  States  Coast  Guard  vessels  records 
of  fog  formation  and  dissipation, 
148w 

United  States  Coast  Survey,  use  of  term 
"gravity,"  28n 

United  States  Revenue  Cutter  Service, 
the  cruises  of  Captain  C.  E.  John- 
ston, over  Labrador  current  and  the 
Gulf  stream,  76;  special  cruises  to 


study  ice  conditions  at  Grand  Bank, 
75 

United  States  Weather  Bureau,  adoption 
of  new  units,  30;  exposures  of  rain 
gauges,  210;  Forecast  division 
storm  classification  by  origin,  87; 
rain  gauges  used  by,  210;  report  on 
international  cloud  observations, 
110;  use  of  term  sleet,  233w;  water 
content  of  snow  available  for  irriga- 
tion, attempt  to  obtain,  225;  water 
content  of  snow,  snow  bins  used  to 
measure,  224 

Units:  and  symbols,  25-32;  absolute,  26; 
British  thermal,  270,  271;  dynamical, 
34;  new,  29;  of  acceleration,  26,  28; 
of  area,  26;  of  density,  26;  of  force, 
26;  of  force  of  atmospheric  pressure, 
27;  of  heat,  34;  large  and  small,  of 
heat,  42;  of  height,  27;  of  height  of 
atmospheric  pressure,  27;  of  momem- 
tum,  26;  of  power,  26,  27;  of  pressure 
force,  28;  of  pressure,  new,  29;  of 
velocity,  26;  of  volume,  26;  of  weight 
of  atmospheric  pressure,  27;  of  work, 
26 

University  of  Arizona:  A.  E.  Douglas, 
identification  of  long  periods  of  tree 
growth,  with  certain  meteorological 
cycles,  216 

University  of  London,  Bjerknes  lecture  on 
storm-tracks,  220 

University  of  Nevada,  Prof.  J.  E.  Church, 
Jr. :  study  of  problem  on  conserva- 
tion of  snow,  225 

Updrafts:  in  cumulus,  cause  of  electrical 
separation,  171;  velocity  of,  in  hail- 
storm, 171 

Upper  inversion,  46 

Utah,  data  on  snow  measure  from,  225 

Valley  breeze,  98 

Valleys  as  catchment  basins  for  slow- 
moving  air,  260 

Valley  winds,  97 

Van  Bemmelen,  Dr.  A.,  cited,  15;  tem- 
perature records  in  balloon  ascents, 
49 

Van  Cleef's  charts  of  highs  and  lows, 
[Figs.  27,  28]  91 

Vapor  and  air:  mixing  equations,  143; 
condensation  of,  on  mountain  slopes, 
[Table  by  Pockels]  212;  content  of, 
determined  by  mixing  ratio,  268; 
on  dust  particles,  160;  pressure  and 
absolute  humidity,  difference  be- 
tween, 267;  water:  see  Water  vapor 
aqueous:  and  vapor  of  saturation, 
[Table  12]  291;  at  saturation,  [Table 
13]  292;  condensation  of,  137;  dissi- 
pation of,  152 

condensed:  and  moist  air,  difference 
between,  261;  effect  of,  262;  forma- 
tion of,  262;  interference  with  free 
radiation,  261 


THE   INDEX 


317 


Vaporization,  latent  heat  of,  39 

Vegard,  L.,  auroral  measurements,  278 

Velocity,  absolute,  and  relative  air  move- 
ment, 55;  and  temperatures  in  cy- 
clones, diagrams  of,  79,  80,  81;  and 
wind  directions,  [normal],  for  January 
and  February,  [Fig.  22]  71;  and  wind 
directions,  normal,  for  July  and 
August,  [Fig.  23]  72;  balance  of 
barometric  gradient  by,  63;  con- 
version of,  [Table  6]  281,  282;  fall  of, 
to  zero,  86;  increase,  and  under- 
lying thermal  structure,  82;  of  air 
in  vortex  of  tornado,  93;  of  cyclonic 
rotation,  determination  of,  62;  of 
propagation  of  sound,  37;  of  sound, 
Newton's  formula  for,  37;  of  torna- 
does, radial  and  tangential,  93;  of 
upper  currents,  change  in,  63;  rela- 
tive, and  relative  air  movement,  55; 
relative  to  direction  of  air  movement, 
57;  reversed,  and  relative  air  move- 
ment, 55;  unit  of,  26 
angular:  of  earth's  rotation,  56,  57; 
earth's  formula  for,  57;  of  earth's 
rotation,  84;  of  point  on  earth's 
surface,  62 

wind:  81;  ascertained  by  use  of  kites,  8; 
effect  of  friction  on,  58;  gradient, 
formula  for,  106;  in  cyclone,  58; 
in  hurricanes  of  1900  and  August, 
1915,  255;  relations  of,  in  strophic 
balance,  83;  symbol  for,  83 

Ventilation,  essentials  of  good,  155 

Verglas:  231,  244 

Vertical  height,  symbol  for,  83 

Vettin,  cited,  112 

Victoria  Nyanza,  observations  at,  49 

Vincent,  J.,  cited,  112 

Vines,  Father,  observations  of  cirrus,  125 

Volcanic:  action  and  atmospheric  dust, 
155;  dust,  color  effect  of,  163;  erup- 
tions and  solar  radiation,  277 

Voltage  of  air:  during  a  snowstorm, 
[Fig.  79]  201;  during  thunderstorm, 
[Fig.  78]  200 

Voltmeter,  static,  use  of,  for  lightning 
records,  190 

Volume :  and  pressure,  temperature  effect 
on,  39;  inversely  as  pressure,  39; 
percentages  of  gases,  22;  unit  of,  26 

Vortices:  cyclonic,  79;  extending  through 
troposphere,  86;  horizontal,  between 
currents,  179;  minor,  in  cross  cur- 
rents, 107;  of  tornadoes,  93;  tornadic, 
movements  of,  92;  tornadic,  as 
secondary  and  localized,  90 

Wada,  Dr.  Y.  of  Korean  Meteorological 
Observatory,  cited,  207,  209 

Walker,  Dr.,  cited,   14 

Walter,  Dr.,  cited,  186 

Ward,  R.  de  C.,  cited,  69,  220 

Washington  Monument;  curves  of  poten- 
tial values  obtained  by  McAdie,  199 


Wasielewski,  W.  v.,  cited,  H2n 
Water:  amount  of,  in  fog,  150;  at  maxi- 
mum density,  temperature  for,  288; 
boiling  point  of,  288;  centrifugal 
action  on,  62;  equivalent  of  snow, 
223;  freezing  point  of,  288;  in  snow 
cover  available  for  irrigation,  225; 
resources,  snow  surveys  for  studies 
in,  225;  supply,  forecasts  from  snow- 
fall, 226;  supply  from  snow  cover, 
225 

vapor:  decrease  with  elevation,  50; 
electrolytic  and  thermal  decomposi- 
tion of,  by  lightning,  189;  in  air,  Qn; 
in  frost  formation,  262;  loss  of,  from 
snow,  225;  molecular  weight  of,  24; 
of  the  atmosphere,  108-136;  per- 
centage in  air,  22,  24;  specific  heat 
at  constant  pressure,  34;  at  constant 
volume,  34 

Waterspout,  93;  at  Chatham  Islands, 
[Fig.  31]  95;  cause  of,  79,  94;  condi- 
tions preceding,  94;  off  Cottage  City, 
Mass.,  [Fig.  30]  94;  on  Hudson 
River,  93;  triple,  94,  95 

Watt,  26,  27 

Waves:  inaudible,  in  thunder,  188;  mo- 
tion, during  thunderstorms,  187; 
of  thunder,  violent,  irregular  series 
of,  188;  reflection,  of  thunder  from 
"interfaces,"  188 

Weather,  abnormal,  and  pressure  areas, 
66;  conditions,  March  28,  1912, 
[Fig.  98]  251;  cyclonic  and  anti- 
cyclonic  control  in  U.  S.,  69;  lore, 
ancient,  1 

Weather  Bureau  at  Galveston,  warning 
of,  instrumental  in  saving  life,  256 

Weber,  W.  E.,  cited,  8,  25 

Weichert,  cited,  28n 

Weightman,  R.  Hanson,  storm  types  of, 
87 

Weight:  molecular,  of  atmospheric  gases, 
24;  of  atmospheric  pressure,  units  of, 
27 

Weights  and  Measures,  International 
Committee  on:  standard  of  normal 
acceleration  adopted  by,  28w 

Weihrauch,  cited,  63 

Weilbach,  cited,  112 

Weinberg,  Borus,  cited,  232n 

Weisner,  A.,  cited,  205 

Wells,  L.  A.,  photographs  of  ice  storms, 
[Fig.  91]  240;  [Fig.  92]  241;  [Fig.  93] 
242;  [Fig.  94]  243 

Wells,  nucleation  of  air,  150 

Wells,  W.  C.,  theory  of  dew,  244 

Welsh,  John,  cited,  10 

West-east  drift,  level  of  maximum,  63 

Westerlies,  prevailing,  97,  101 

Westerly  component  of  air  current,  82 

West  Indian  hurricanes:  87,  88;  move- 
ments, 88;  of  1915  rainfall  distribu- 
tion, [Fig.  104]  257;  type  as  forecast, 
125 


318 


THE   INDEX 


West  Indian  type  of  storm,  87 

"West  wind  drift,"  74 

Whipple,  cited,  28n 

Wilkes,  Capt.  Charles,  U.  S.  N.,  early 
map  of  winds,  97 

William  III,  King,  equipment  of  Halley's 
voyage  by,  97 

Willson,  G.  H.,  cited,  69w 

Wilson,  Alexander,  cited,  8 

Wilson,  cited,   148 

Wind:  97-107;  absolute  motion  of  radi- 
ally directed,  at  surface  of  rotating 
system,  [Fig.  19]  62;  accidental,  98; 
action  and  seasonal  curve  of  snow- 
fall, 225;  agencies  controlling,  98; 
and  attrition  of  atmospheric  dust, 
155;  and  barometric  gradient,  rota- 
tion between,  63;  and  pressure  dis- 
tribution, equivalence  of,  law  of,  84; 
an  important  factor  in  melting,  229; 
anti-trade,  height  of,  51;  Aristotle's 
treatise  on,  2;  boreal,  104;  "brave 
west,"  100;  change  of,  near  center  of 
New  Orleans  storm  of  September, 
1915,  [Fig.  103]  256;  change,  sudden, 
in  ice  storm,  238;  Chinook,  104; 
conditions  which  produce  ice  storms, 
234;  continental,  98;  cyclonic,  98; 
cyclostrophic,  84;  direction,  resulting 
from  pressure  distribution,  104; 
distribution  and  ice  storm  inversions 
of  temperature,  234;  distribution  of, 
in  cyclones  and  anticyclones,  [Fig. 
24]  79;  duration,  104;  duration  and 
floods,  246;  earliest  maps  and  charts 
of,  97;  east,  south  of  trough  of  low 
pressure,  70;  effect  of  radially  di- 
rected, upon  a  rotating  system, 
[Fig.  17]  61;  effect  of  radially  di- 
rected, upon  a  system  at  rest, 
[Fig.  16]  61;  first  scientific  designa- 
tion of,  1;  foehn:  see  Foehn;  geo- 
strophic,  84;  gradient,  106;  gusty, 
and  temperature,  relation  of,  239; 
Helm,  151;  high,  and  snows  in  Mid- 
dle Atlantic  and  New  England 
states,  90;  "hot,"  of  plains  states, 
104;  influence  on  storm  track,  220; 
influenced  by  topography,  102; 
Koppen's  charts  of,  70;  [Fig.  22]  71; 
[Fig.  23]  72;  light,  in  central  region 
of  anticyclone,  84;  lines  of  converg- 
ence and  divergence  of,  indicating 
cyclonic  and  anticyclonic  motion, 
220;  local,  98,  102;  monsoon:  see 
Monsoons;  mountain  and  valley, 
102;  movement  of  soil  by,  155; 
movement,  rate  of  evaporation  and, 
241;  movement,  world  maps  of,  97; 
northwest,  gusty,  dying,  in  relation 


to  frost,  264;  of  Antarctic  region, 
105;  permanent,  97;  planetary,  97, 
98;  prevailing,  atmospheric  pressure 
at  right  and  left  of,  63;  relations  be- 
tween, and  isallobars,  90;  relative 
motion  of  radially  directed,  at  sur- 
face of  rotating  system,  [Fig.  18]  62; 
rapid  falling  off,  in  stratosphere,  86; 
rose  of  Babylonians,  1;  shift  line, 
237;  seasonal,  cause  of,  98;  shallow- 
ness  and  depth  of,  105;  steady, 
earliest  knowledge  of,  54;  storm 
types  of,  97;  strength,  varies  with 
height,  107;  summer  and  winter 
variations,  106;  surface,  and  sea- 
level  pressure  during  dry  winter 
months,  [Fig.  20]  68;  surface,  and 
sea-level  pressure  during  wet  winter 
months,  [Fig.  21]  68;  systems,  97; 
system  of  Atlantic,  first  knowledge 
of,  97;  trade:  see  Trades;  terrestrial, 
98;  Tower  of  the,  at  Athens,  1; 
types  of,  97,  104;  valley,  97 
direction:  above  trade  winds,  48;  and 
temperature,  104;  in  cyclone,  71; 
from  pressure  distribution,  104; 
normal,  and  velocities  for  January 
and  February,  [Fig.  22]  71;  normal, 
and  velocities  for  July  and  August, 
[Fig.  23]  72 

scale:  Beaufort,  290;  equivalent  values 
for,  290;  formula  for,  290;  [Table  11] 
290 

velocity:  and  gradient,  relation  be- 
tween, 58;  and  pressure  and  tem- 
perature distribution,  8;  ascertained 
by  use  of  kites,  8;  at  time  of  Gal- 
veston  flood,  253;  change  with 
height,  formula  for,  84;  effect  of 
friction  on,  58;  first  attempt  to 
measure,  8;  in  cyclone,  58;  in  hurri- 
canes of  1900  and  August,  1915,  255; 
level  of  greatest,  81;  symbol  for,  83: 
see  also  Velocity 

Winter:  sun,  heating  effect  of,  273; 
trades  of  Indian  Ocean,  100;  type  of 
inversion,  [Fig.  105]  261 

Wireless  receivers,  storm  predictions  on, 
189;  use  of,  for  lightning  records,  190 

Wolff  sunspot  numbers,  276 

Work,  unit  of,  26 

Xenon  in  atmosphere,  7 
Year,  sidereal,  32 

Zero,  new,  36 

Zi-ka-wei   Observatory:   study  of  upper 
clouds  for  forecasting  purposes,   127 
Zones,  dry,  and  trades,  217 


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DEC   *-6   1934 


OCT    2    1942 


APR    13  t943 


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